Hide NavigationRegion2D's debug instance instead of freeing it, and hide it when navigation_polygon is set to null.

[Daylily-Zeleen]

An alternative of #99948.

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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.

@smix8 I can't find the " navigation_polygon.is_valid() check" that can be removed. Could your point it out?

[Repiteo]

Thanks!