fix: NPE while creating a policies copy (appsmithorg#36172)

## Description
PR to handle the empty collection and null values for policies. We
expect web should never end up in this state but we have received enough
evidences that because of broken functionality in GAC we may end up in
this scenario. One such example is
appsmithorg#36063

/test all

### 🔍 Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!CAUTION]
> 🔴 🔴 🔴 Some tests have failed.
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/10767073729>
> Commit: e225d54
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=10767073729&attempt=2&selectiontype=test&testsstatus=failed&specsstatus=fail"
target="_blank">Cypress dashboard</a>.
> Tags: @tag.All
> Spec: 
> The following are new failures, please fix them before merging the PR:
<ol>
>
<li>cypress/e2e/Regression/ClientSide/Debugger/Widget_property_navigation_spec.ts
>
<li>cypress/e2e/Regression/ClientSide/OneClickBinding/PropertyControl_spec.ts</ol>
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/identified-flaky-tests-65890b3c81d7400d08fa9ee3?branch=master"
target="_blank">List of identified flaky tests</a>.
> <hr>Mon, 09 Sep 2024 07:01:41 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [x] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit


- **Bug Fixes**
- Improved handling of user permissions to prevent potential errors
related to null values in policies.
- Enhanced robustness in policy management methods to avoid null-related
errors when retrieving existing policies.
- **Tests**
- Added unit tests for user permissions handling and policy management
to ensure robust behavior under various conditions.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/server/appsmith-server/src/main/java/com/appsmith/server/repositories/ce/BaseAppsmithRepositoryCEImpl.java

@@ -416,7 +416,8 @@ public Mono<T> setUserPermissionsInObject(T obj, Set<String> permissionGroups) {
         Set<String> permissions = new HashSet<>();
         obj.setUserPermissions(permissions);
 
-        Set<Policy> policies = new HashSet<>(obj.getPolicies());
+        Set<Policy> existingPolicies = obj.getPolicies();
+        final Set<Policy> policies = new HashSet<>(existingPolicies == null ? Set.of() : existingPolicies);
         if (CollectionUtils.isEmpty(policies) || permissionGroups.isEmpty()) {
             return Mono.just(obj);
         }

app/server/appsmith-server/src/main/java/com/appsmith/server/solutions/ce/PolicySolutionCEImpl.java

@@ -69,7 +69,8 @@ public <T extends BaseDomain> T addPoliciesToExistingObject(@NonNull Map<String,
             policyMap1.put(entry.getKey(), policy);
         }
 
-        final Set<Policy> policies = new HashSet<>(obj.getPolicies());
+        Set<Policy> existingPolicies = obj.getPolicies();
+        final Set<Policy> policies = new HashSet<>(existingPolicies == null ? Set.of() : existingPolicies);
 
         // Append the user to the existing permission policy if it already exists.
         for (Policy policy : policies) {
@@ -102,7 +103,8 @@ public <T extends BaseDomain> T removePoliciesFromExistingObject(Map<String, Pol
             policyMap1.put(entry.getKey(), entry.getValue());
         }
 
-        Set<Policy> policies = new HashSet<>(obj.getPolicies());
+        Set<Policy> existingPolicies = obj.getPolicies();
+        final Set<Policy> policies = new HashSet<>(existingPolicies == null ? Set.of() : existingPolicies);
         // Remove the user from the existing permission policy if it exists.
         for (Policy policy : policies) {
             String permission = policy.getPermission();

app/server/appsmith-server/src/test/java/com/appsmith/server/repositories/ce/BaseAppsmithRepositoryImplTest.java

@@ -0,0 +1,52 @@
+package com.appsmith.server.repositories.ce;
+
+import com.appsmith.external.models.BaseDomain;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.UUID;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+class BaseAppsmithRepositoryImplTest {
+
+    BaseAppsmithRepositoryCEImpl<TestClass> baseAppsmithRepositoryImpl;
+
+    @BeforeEach
+    public void setup() {
+        baseAppsmithRepositoryImpl = new BaseAppsmithRepositoryCEImpl<>() {};
+    }
+
+    class TestClass extends BaseDomain {}
+
+    @Test
+    void testSetUserPermissionsInObject_whenPoliciesIsEmptySet_emptyCollectionValueIsSet() {
+        // Test the method setPoliciesInObject when the policies are null
+        // The method should set an empty collection value in the object
+        // The method should return the object
+        TestClass obj = baseAppsmithRepositoryImpl
+                .setUserPermissionsInObject(new TestClass(), null)
+                .block();
+        assertNotNull(obj);
+        Assertions.assertEquals(0, obj.getPolicies().size());
+    }
+
+    @Test
+    void testSetUserPermissionsInObject_whenPoliciesIsNull_nullPoliciesAreSet() {
+        // Test the method setPoliciesInObject when the policies are empty
+        // The method should set an empty collection value in the object
+        // The method should return the object
+        TestClass obj = new TestClass();
+        obj.setPolicies(null);
+        Set<String> permissionGroups = new HashSet<>();
+        permissionGroups.add(UUID.randomUUID().toString());
+        obj = baseAppsmithRepositoryImpl
+                .setUserPermissionsInObject(obj, permissionGroups)
+                .block();
+        assertNotNull(obj);
+        Assertions.assertNull(obj.getPolicies());
+    }
+}

app/server/appsmith-server/src/test/java/com/appsmith/server/solutions/ce/PolicySolutionCEImplTest.java

@@ -32,6 +32,18 @@ private static class TestClass extends BaseDomain {
         TestClass() {}
     }
 
+    @Test
+    void testAddNewPoliciesToNullPoliciesObject() {
+        TestClass obj = new TestClass();
+        obj.setPolicies(null);
+        Map<String, Policy> policyMap = new HashMap<>();
+        policyMap.put("read", new Policy("read", new HashSet<>(Set.of("group1"))));
+
+        BaseDomain result = policySolution.addPoliciesToExistingObject(policyMap, obj);
+
+        assertTrue(result.getPolicies().containsAll(policyMap.values()));
+    }
+
     @Test
     void testAddNewPoliciesToEmptyObject() {
         BaseDomain obj = new TestClass(); // Assuming BaseDomain has a default empty set of policies.