BUGFIX: redis vector store overwrites falsey metadata (#13652)

- **Description:** This commit fixed the problem that Redis vector store
will change the value of a metadata from 0 to empty when saving the
document, which should be an un-intended behavior.
  - **Issue:** N/A
  - **Dependencies:** N/A

libs/langchain/langchain/vectorstores/redis/base.py

@@ -1381,7 +1381,7 @@ def _prepare_metadata(metadata: Dict[str, Any]) -> Dict[str, Any]:
 
     clean_meta: Dict[str, Union[str, float, int]] = {}
     for key, value in metadata.items():
-        if not value:
+        if value is None:
             clean_meta[key] = ""
             continue