nixpkgs-plugin-update: fall back on empty refs

Treat empty GitHub GraphQL tag refs as inconclusive.

Use the existing tag feed/git fallback path instead of treating an empty API result as authoritative no-tags state.

pkgs/development/python-modules/nixpkgs-plugin-update/nixpkgs-plugin-update/src/nixpkgs_plugin_update/__init__.py

@@ -429,7 +429,7 @@ class RepoGitHub(Repo):
 
             recent_tags = [node["name"] for node in repo["refs"]["nodes"]]
             if not recent_tags:
-                return None
+                return self._get_latest_tag_from_fallbacks()
 
             latest_tag = first_release_tag(recent_tags)
             return latest_tag if latest_tag is not None else recent_tags[0]