fix chat and thumbnail bug (#2803)

### What problem does this PR solve?

1. fix white screen issue when chat response
2. thumbnail bug when document not support

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

---------

Co-authored-by: chongchuanbing <chongchuanbing@gmail.com>

api/db/services/file_service.py

@@ -358,8 +358,10 @@ class FileService(CommonService):
                 doc_id = get_uuid()
 
                 img = thumbnail_img(filename, blob)
-                thumbnail_location = f'thumbnail_{doc_id}.png'
-                STORAGE_IMPL.put(kb.id, thumbnail_location, img)
+                thumbnail_location = ''
+                if img is not None:
+                    thumbnail_location = f'thumbnail_{doc_id}.png'
+                    STORAGE_IMPL.put(kb.id, thumbnail_location, img)
 
                 doc = {
                     "id": doc_id,