incrementalDelivery: fix iterable streaming with errors

Replicates graphql/graphql-js@065a343

Author: Cito

src/graphql/execution/execute.py

@@ -1702,7 +1702,6 @@ def execute_stream_field(
             label, item_path, None, parent_context, self
         )
         completed_item: Any
-        completed_items: Any
         try:
             try:
                 if self.is_awaitable(item):
@@ -1771,6 +1770,7 @@ async def await_completed_item() -> Any:
             async_payload_record.add_items(None)
             return async_payload_record
 
+        completed_items: AwaitableOrValue[Optional[List[Any]]]
         if self.is_awaitable(complete_item):
 
             async def await_completed_items() -> Optional[List[Any]]:

tests/execution/test_stream.py

@@ -992,10 +992,7 @@ async def handles_error_thrown_in_complete_value_after_initial_count_is_reached(
         )
 
         async def scalar_list(_info):
-            await sleep(0)
-            yield friends[0].name
-            await sleep(0)
-            yield {}
+            return [friends[0].name, {}]
 
         result = await complete(document, {"scalarList": scalar_list})
         assert result == [