Free debug mesh when NavigationRegion2D's navigation_polygon is set to null.

[Daylily-Zeleen]

Make NavigationRegion2D(free debug elements) similar to NavigationRegion3D(make debug elements invisible).

[smix8]

I guess we could remove the navigation_polygon.is_valid() check in the notification draw function instead as that should clear the debug canvas item instance and the debug mesh without freeing the entire rendering RIDs. Although there are likely some missing checks then in the later functions.

The NavigationRegion3D also only hides the visibility of the debug instance instead of freeing the RIDs.

[smix8]

As mentioned I would prefer if the rendering RIDs could be kept someway but if that turns out too much change is needed I am fine merging at is.

[akien-mga]

Superseded by #100006.