langchain: fix OutputParserException (#15914) (#15916)

**Description:**

Fixes OutputParserException thrown by the output_parser when 'query' is
'Null'.

Replace this entire comment with:
- **Description:** Current implentation of output_parser throws
OutputParserException if the response from the LLM contains `query:
null`. This unfortunately happens for my use case. And since there is no
way to modify the prompt used in SelfQueryRetriever, then we have to fix
it here, so it doesn't crash.
  - **Issue:** https://github.com/langchain-ai/langchain/issues/15914

Didn't run tests. `make test` is not working. There is no `test` rule in
the `Makefile`.

Co-authored-by: Jan Horcicka <jhorcick@amazon.com>

libs/langchain/langchain/chains/query_constructor/base.py

@@ -48,7 +48,7 @@ class StructuredQueryOutputParser(BaseOutputParser[StructuredQuery]):
             expected_keys = ["query", "filter"]
             allowed_keys = ["query", "filter", "limit"]
             parsed = parse_and_check_json_markdown(text, expected_keys)
-            if len(parsed["query"]) == 0:
+            if parsed["query"] is None or len(parsed["query"]) == 0:
                 parsed["query"] = " "
             if parsed["filter"] == "NO_FILTER" or not parsed["filter"]:
                 parsed["filter"] = None