fix: stop rerank by model when search result is empty (#4203)

### What problem does this PR solve?


stop rerank by model when search result is empty, otherwise rerank may
raise an error (qwen).

### Type of change

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

Co-authored-by: 刘博 <liubo@ynby.cn>

rag/nlp/search.py

         ranks["total"] = sres.total
 
         if page <= RERANK_PAGE_LIMIT:
-            if rerank_mdl:
+            if rerank_mdl and sres.total > 0:
                 sim, tsim, vsim = self.rerank_by_model(rerank_mdl,
                     sres, question, 1 - vector_similarity_weight, vector_similarity_weight)
             else: