fix: Keep module state when editing students

src/main/java/seedu/address/logic/commands/EditCommand.java

@@ -146,7 +146,7 @@ public static class EditStudentDescriptor {
         private Address address;
         private Set<Tag> tags;
 
-        private List<ModuleCode> modules = new ArrayList<>();
+        private List<ModuleCode> modules;
 
         private List<ModuleTiming> moduleTimings = new ArrayList<>();
 
@@ -167,7 +167,7 @@ public EditStudentDescriptor(EditStudentDescriptor toCopy) {
         }
 
         public void setModules(List<ModuleCode> modules) {
-            this.modules = new ArrayList<>(modules);
+            this.modules = (modules != null) ? new ArrayList<>(modules) : null;
         }
 
         public void setModuleTimings(List<ModuleTiming> modules) {

src/test/java/seedu/address/logic/parser/AddressBookParserTest.java

@@ -57,6 +57,9 @@ public void parseCommand_delete() throws Exception {
     public void parseCommand_edit() throws Exception {
         Student student = new StudentBuilder().build();
         EditStudentDescriptor descriptor = new EditStudentDescriptorBuilder(student).build();
+        // (taufiq): I have to set this to null because we need to differentiate
+        // between presence/absence of module input
+        descriptor.setModules(null);
         EditCommand command = (EditCommand) parser.parseCommand(EditCommand.COMMAND_WORD + " "
                 + INDEX_FIRST_STUDENT.getOneBased() + " " + StudentUtil.getEditStudentDescriptorDetails(descriptor));
         assertEquals(new EditCommand(INDEX_FIRST_STUDENT, descriptor), command);