Fix: docx image exceptions. (#6839)

### What problem does this PR solve?

Close #6784

### Type of change

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

rag/app/naive.py

         if not img:
             return None
         img = img[0]
-        embed = img.xpath('.//a:blip/@r:embed')[0]
+        embed = img.xpath('.//a:blip/@r:embed')
+        if not embed:
+            return None
+        embed = embed[0]
         related_part = document.part.related_parts[embed]
         try:
             image_blob = related_part.image.blob