fix: null check error when using withConverter and returning null f…

…rom `fromFirestore` (#11224)

* fix: null check error when using `withConverter` and returning null from `fromFirestore`

* add e2e test

* fix: generated firebase files

* refactor(cloud_firestore): remove issue number from test

* refactor(cloud_firestore): revert GoogleService-Info.plist

---------

Co-authored-by: Russell Wheatley <russellwheatley85@gmail.com>

packages/cloud_firestore/cloud_firestore/example/integration_test/collection_reference_e2e.dart

@@ -169,6 +169,32 @@ void runCollectionReferenceTests() {
         },
         timeout: const Timeout.factor(3),
       );
+
+      testWidgets(
+        'returning null from `fromFirestore` should not throw a null check error',
+        (_) async {
+          final foo = await initializeTest('foo');
+          await foo.add({'value': 42});
+          final fooConverter = foo.withConverter(
+            fromFirestore: (_, __) => null,
+            toFirestore: (_, __) => {}, // unused
+          );
+
+          final fooConverterSnapshot = fooConverter.snapshots();
+
+          await expectLater(
+            fooConverterSnapshot,
+            emits(
+              // ignore: prefer_void_to_null
+              isA<QuerySnapshot<Null>>().having((e) => e.docs, 'docs', [
+                // ignore: prefer_void_to_null
+                isA<QueryDocumentSnapshot<Null>>()
+                    .having((e) => e.data(), 'data', null)
+              ]),
+            ),
+          );
+        },
+      );
     });
   });
 }

packages/cloud_firestore/cloud_firestore/lib/src/query_document_snapshot.dart

@@ -52,5 +52,5 @@ class _WithConverterQueryDocumentSnapshot<T extends Object?>
   bool get exists => true;
 
   @override
-  T data() => super.data()!;
+  T data() => super.data() as T;
 }