Ignore exceptions when no index ahead. (#5047)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

rag/nlp/search.py

@@ -449,6 +449,8 @@ class Dealer:
         return res
 
     def all_tags(self, tenant_id: str, kb_ids: list[str], S=1000):
+        if not self.docStoreConn.indexExist(index_name(tenant_id), kb_ids[0]):
+            return []
         res = self.dataStore.search([], [], {}, [], OrderByExpr(), 0, 0, index_name(tenant_id), kb_ids, ["tag_kwd"])
         return self.dataStore.getAggregation(res, "tag_kwd")