fix bug with missing UniqueID.number field in PageProperty.

I ran into a bug today where my notion-client was failing to deserialize a Page containing a PageProperty.UniqueID.unique_id field
whose UniqueIDPropertyValue.number field was null. 99.999% of the time UniqueIDPropertyValue.number is non-null, but it can be null
for instance in the case where you use one of Notion's templates to create a Task Page with a null UniqueIDPropertyValue.number
field, but that's OK because the Task Page will always be copied to make a new Task and the unique ID field will be filled in with
some value.

This change to make number an Option<Number> instead of just Number reflects the fact that it could be null, though it is very
unlikely.

src/endpoints/search/tests.rs

@@ -32,3 +32,12 @@ fn test_search_by_title_200() {
     ));
     assert!(result.is_ok())
 }
+
+#[test]
+fn test_search_by_title_failure() {
+    let result = serde_json::from_str::<SearchByTitleResponse>(include_str!(
+        "tests/search_by_title_failure_with_missing_unique_id.json"
+    ));
+    dbg!(&result);
+    assert!(result.is_ok())
+}

src/objects/page.rs

@@ -241,7 +241,9 @@ pub struct SelectPropertyValue {
 
 #[derive(Serialize, Deserialize, Debug, Eq, PartialEq, Clone)]
 pub struct UniqueIDPropertyValue {
-    pub number: Number,
+    #[serde(default)]
+    #[doc = "This field is null only in extremely rare cases, for instance when it's being used as a Template for a Task in Notion's Project Management Template: https://www.notion.so/templates/notion-projects-and-tasks"]
+    pub number: Option<Number>,
     pub prefix: Option<String>,
 }