Emit did_change_signal when no actions are mapped

[ryan-linehan]

Is your feature request related to a problem? Please describe. When an action is updated and there are no actions mapped yet the did_change_signal does not emit. In my game I would like players to be able to add optional key binds for actions.

Describe the solution you'd like When adding a mapping to an input but there are no inputs already mapped to the action the did_change_signal emits

Additional context Making this change to _update_input_for_action seems to make it work:

An example of the context it might be useful in:

https://github.com/nathanhoad/godot_input_helper/assets/12516772/01ee92ff-92f8-4c8c-881d-508e8a2d233a