Fix test to send empty string instead of empty file

tests/test_optional_file_list.py

@@ -32,7 +32,7 @@ def test_optional_bytes_list_send_empty_str():
     client = TestClient(app)
     response = client.post(
         "/files",
-        files=[("files", b"")],
+        data={"files": ""},
     )
     assert response.status_code == 200
     assert response.json() == {"files_count": 1, "sizes": [0]}