Fix json key output parser in partial (streaming) mode (#12332)

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libs/langchain/langchain/output_parsers/openai_functions.py

@@ -130,7 +130,9 @@ class JsonKeyOutputFunctionsParser(JsonOutputFunctionsParser):
     """The name of the key to return."""
 
     def parse_result(self, result: List[Generation], *, partial: bool = False) -> Any:
-        res = super().parse_result(result)
+        res = super().parse_result(result, partial=partial)
+        if partial and res is None:
+            return None
         return res.get(self.key_name) if partial else res[self.key_name]