fix: preserve pgvector None sentinel on zero top_k

query_external_vector_document_ids must return None when on the
pgvector path before applying the top_k<=0 empty-list guard.

src/crud/document.py

@@ -228,12 +228,12 @@ async def query_external_vector_document_ids(
         empty list when the external store has no results,
         or None when the pgvector (DB-only) path should be used instead.
     """
-    if top_k <= 0:
-        return []
-
     if _uses_pgvector():
         return None
 
+    if top_k <= 0:
+        return []
+
     external_vector_store = get_external_vector_store()
     if external_vector_store is None:
         return []