Fix: empty query issue. (#5830)

### What problem does this PR solve?

#5214

### Type of change

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

rag/nlp/query.py

         otxt = txt
         for r, p in patts:
             txt = re.sub(r, p, txt, flags=re.IGNORECASE)
-        if not otxt:
+        if not txt:
             txt = otxt
         return txt