fix(ui_firestore): null-assert count for aggregate query (#246)

Author: lesnitsky

packages/firebase_ui_firestore/lib/src/table_builder.dart

@@ -871,7 +871,7 @@ class _Source extends DataTableSource {
 
   @override
   int get rowCount {
-    if (_aggregateSnapshot?.count != null) return _aggregateSnapshot!.count;
+    if (_aggregateSnapshot?.count != null) return _aggregateSnapshot!.count!;
     // Emitting an extra item during load or before reaching the end
     // allows the DataTable to show a spinner during load & let the user
     // navigate to next page