Fix: loopitem None issue. (#12166)

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/services/document_service.py

@@ -701,6 +701,8 @@ class DocumentService(CommonService):
                     v = [v]
                 for vv in v:
                     if vv not in meta[k]:
+                        if isinstance(vv, list) or isinstance(vv, dict):
+                            continue
                         meta[k][vv] = []
                     meta[k][vv].append(doc_id)
         return meta