Fix subscribe() iterator

Author: Cito

src/graphql/execution/incremental_publisher.py

@@ -397,8 +397,7 @@ async def subscribe(
 
                 if result is not None:
                     yield result
-
-                if not is_done:
+                else:
                     await self._resolve.wait()
         finally:
             close_async_iterators = []

tests/execution/test_defer.py

@@ -782,7 +782,6 @@ async def handles_async_non_nullable_errors_thrown_in_deferred_fragments():
 
     @pytest.mark.asyncio()
     async def returns_payloads_in_correct_order():
-        return  # TODO: hangs
         document = parse(
             """
             query HeroNameQuery {
@@ -836,7 +835,6 @@ async def returns_payloads_in_correct_order():
 
     @pytest.mark.asyncio()
     async def returns_payloads_from_synchronous_data_in_correct_order():
-        return  # TODO: hangs
         document = parse(
             """
             query HeroNameQuery {