Return 4xx when file download storage blob is missing (#15371)

## Summary

Fixes [#15369](https://github.com/infiniflow/ragflow/issues/15369) —
`GET /api/v1/files/<file_id>` calls `make_response(None)` when both
primary and fallback storage lookups return empty, causing HTTP 500.

## Related Issue

Fixes #15369

## Change Type

- [x] Bug fix
- [x] Regression tests

## What Changed

- **`file_api.download()`** — after fallback `STORAGE_IMPL.get`, return
`get_error_data_result(message="This file is empty.")` when `not blob`,
matching document REST download semantics.

## Files Changed

| File | Change |
|------|--------|
| `api/apps/restful_apis/file_api.py` | Empty-blob guard before
`make_response()` |
| `test/testcases/test_web_api/test_file_app/test_file_routes_unit.py` |
Regression test |

## Validation

```bash
cd /root/gittensor/ragflow
pytest test/testcases/test_web_api/test_file_app/test_file_routes_unit.py::test_download_missing_blob_returns_error -v
pytest test/testcases/test_web_api/test_file_app/test_file_routes_unit.py::test_download_falls_back_to_document_storage -v
```

## Test Plan

- [x] Both storage paths empty → `"This file is empty."` (no
`make_response(None)`)
- [x] Existing fallback success test still passes
- [ ] CI green