fix use empty list default to empty list

Co-authored-by: Stephen Finucane <[email protected]>

sphinxcontrib/openapi/openapi31.py

@@ -302,7 +302,7 @@ def _get_type_from_schema(schema):
             dtype = schema["type"]
         else:
             dtype = set()
-            for t in schema.get("anyOf", schema.get("allOf")):
+            for t in schema.get("anyOf", schema.get("allOf", [])):
                 if "format" in t.keys():
                     dtype.add(t["format"])
                 else: