Fix: empty tag field issue. (#6103)

### What problem does this PR solve?

#6102

### Type of change

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

rag/nlp/search.py

@@ -258,6 +258,8 @@ class Dealer:
         q_denor = np.sqrt(np.sum([s*s for t,s in query_rfea.items() if t != PAGERANK_FLD]))
         for i in search_res.ids:
             nor, denor = 0, 0
+            if not search_res.field[i].get(TAG_FLD):
+                continue
             for t, sc in eval(search_res.field[i].get(TAG_FLD, "{}")).items():
                 if t in query_rfea:
                     nor += query_rfea[t] * sc