Guard missing storage blobs on preview and image endpoints (#15366)

Fixes [#15365](https://github.com/infiniflow/ragflow/issues/15365) —
`get_document_image()` and document preview call `make_response(None)`
when storage returns no bytes, causing HTTP 500.

api/apps/restful_apis/document_api.py

@@ -1924,6 +1924,8 @@ async def get(doc_id):
 
         b, n = File2DocumentService.get_storage_address(doc_id=doc_id)
         data = await thread_pool_exec(settings.STORAGE_IMPL.get, b, n)
+        if not data:
+            return get_data_error_result(message="This file is empty.")
         response = await make_response(data)
 
         ext = re.search(r"\.([^.]+)$", doc.name.lower())