### What problem does this PR solve?
stack：
```
2025-06-26 17:22:24,739 ERROR 1609 list index out of range
Traceback (most recent call last):
File "/ragflow/.venv/lib/python3.10/site-packages/flask/app.py", line 880, in full_dispatch_request
rv = self.dispatch_request()
File "/ragflow/.venv/lib/python3.10/site-packages/flask/app.py", line 865, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args) # type: ignore[no-any-return]
File "/ragflow/api/utils/api_utils.py", line 298, in decorated_function
return func(*args, **kwargs)
File "/ragflow/api/apps/sdk/session.py", line 472, in list_session
print(conv["reference"][message_num])
IndexError: list index out of range
```
### Type of change
- [ ] 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):
daf6c82066