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

test/testcases/test_web_api/test_file_app/test_file_routes_unit.py

@@ -327,6 +327,22 @@ def test_download_falls_back_to_document_storage(monkeypatch):
     assert res.headers["ext"] == "txt"
 
 
+@pytest.mark.p2
+def test_download_missing_blob_returns_error(monkeypatch):
+    module = _load_file_api_module(monkeypatch)
+    storage_calls = []
+
+    def _get(bucket, location):
+        storage_calls.append((bucket, location))
+        return None
+
+    monkeypatch.setattr(module.settings, "STORAGE_IMPL", SimpleNamespace(get=_get))
+    res = _run(module.download("tenant1", "file1"))
+
+    assert storage_calls == [("bucket1", "path1"), ("bucket2", "path2")]
+    assert res["message"] == "This file is empty."
+
+
 @pytest.mark.p2
 def test_parent_and_ancestors_use_new_routes(monkeypatch):
     module = _load_file_api_module(monkeypatch)