Gio.Application: Do not pass user_data when it's unset for add_action_entries

Python is sensitive to the number of required arguments. Callbacks connected by action.connect('activate', activate) will not receive the additional None as user_data argument, while action.connect('activate', activate, None) does. For most of time user_data is unnecessary, and passing None unconditionally makes existing code using action.connect('activate', activate) hard to adopt add_action_entries function, vice versa.