fix: add fallback instance title

otherwise the hero bar can be hidden altogether

src/wuttaweb/views/master.py

@@ -2000,7 +2000,7 @@ class MasterView(View):
         Default logic returns the value from ``str(instance)``;
         subclass may override if needed.
         """
-        return str(instance)
+        return str(instance) or "(no title)"
 
     def get_action_url(self, action, obj, **kwargs):
         """