add isCompletedIterator to AsyncPayloadRecord to track completed iterators

# Conflicts:
#	spec/Section 6 -- Execution.md

Author: robrichard

spec/Section 6 -- Execution.md

@@ -386,13 +386,13 @@ YieldSubsequentPayloads(subsequentPayloads):
 - Let {record} be the first item in {subsequentPayloads} with a completed
   {dataExecution}.
   - Remove {record} from {subsequentPayloads}.
-  - Let {payload} be the completed result returned by {dataExecution}.
-  - If {payload} is {null}:
+  - If {isCompletedIterator} on {record} is {true}:
     - If {subsequentPayloads} is empty:
       - Yield a map containing a field `hasNext` with the value {false}.
       - Return.
     - If {subsequentPayloads} is not empty:
       - Continue to the next record in {subsequentPayloads}.
+  - Let {payload} be the completed result returned by {dataExecution}.
   - If {record} is not the final element in {subsequentPayloads}:
     - Add an entry to {payload} named `hasNext` with the value {true}.
   - If {record} is the final element in {subsequentPayloads}:
@@ -681,6 +681,8 @@ All Async Payload Records are structures containing:
 - {path}: a list of field names and indices from root to the location of the
   corresponding `@defer` or `@stream` directive.
 - {iterator}: The underlying iterator if created from a `@stream` directive.
+- {isCompletedIterator}: a boolean indicating the payload record was generated
+  from an iterator that has completed.
 - {errors}: a list of field errors encountered during execution.
 - {dataExecution}: A result that can notify when the corresponding execution has
   completed.
@@ -863,6 +865,7 @@ variableValues, subsequentPayloads):
 - Let {dataExecution} be the asynchronous future value of:
   - Wait for the next item from {iterator}.
   - If an item is not retrieved because {iterator} has completed:
+    - Set {isCompletedIterator} to {true} on {streamRecord}.
     - Return {null}.
   - Let {payload} be an unordered map.
   - Let {item} be the item retrieved from {iterator}.