Fix neo4j type error on missing constraint information (langchain-ai#…

…24177)

If you use `refresh_schema=False`, then the metadata constraint doesn't
exist. ATM, we used default `None` in the constraint check, but then
`any` fails because it can't iterate over None value

libs/community/langchain_community/graphs/neo4j_graph.py

@@ -555,10 +555,11 @@ def add_graph_documents(
                     el["labelsOrTypes"] == [BASE_ENTITY_LABEL]
                     and el["properties"] == ["id"]
                     for el in self.structured_schema.get("metadata", {}).get(
-                        "constraint"
+                        "constraint", []
                     )
                 ]
             )
+
             if not constraint_exists:
                 # Create constraint
                 self.query(