Batch operations fail when listening to document changes

[ccrusius]

(May be a duplicate of #182.)

When I batch create two documents while listening for document changes I see null document data coming in. The following test fails:

import 'package:fake_cloud_firestore/fake_cloud_firestore.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  test('Batch with doc change subscription', () async {
    final db = FakeFirebaseFirestore();
    final sub = db.collection('docs').snapshots().listen((querySnapshot) {
      for (final change in querySnapshot.docChanges) {
        debugPrint(
            'Change: ${change.doc.id} ${change.type} ${change.doc.data()}');
      }
    });
    addTearDown(sub.cancel);

    final batch = db.batch();
    batch.set(db.collection('docs').doc('1'), <String, dynamic>{});
    batch.set(db.collection('docs').doc('2'), <String, dynamic>{});
    await batch.commit();
    await pumpEventQueue();
  });
}

with

Change: 1 DocumentChangeType.added null
Change: 2 DocumentChangeType.added null
package:fake_cloud_firestore/src/mock_query_document_snapshot.dart 16:27  MockQueryDocumentSnapshot.data
package:fake_cloud_firestore/src/fake_query_with_parent.dart 45:27        _snapshotEquals
... (snip) ...
Null check operator used on a null value

This works if changes are non-batched, or if there's no listen in place. Tried with latest master, fails there too.