fix add local vision llm error when cannot download test pic (#1732)

### What problem does this PR solve?

#1726   fix add local vision llm  error when cannot download test pic

### Type of change

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

Co-authored-by: Zhedong Cen <cenzhedong2@126.com>

api/apps/llm_app.py

                 if not tc:
                     raise Exception(m)
             else:
-                raise ConnectionError("fail to download the test picture")
+                pass
         except Exception as e:
             msg += f"\nFail to access model({llm['llm_name']})." + str(e)
     else: