Fix "Navigation map synchronization error" when using NavigationRegion2D

[mrTag]

The constructor of NavigationPolygon instantiates its navigation_mesh variable, preventing the NavigationPolygon::get_navigation_mesh() function from initializing the navigation_mesh object correctly, leading to this error:

modules/navigation/nav_region.cpp:128 - Navigation map synchronization error. Attempted to update a navigation region with a navigation mesh that uses a `cell_size` of 1 while assigned to a navigation map set to a `cell_size` of 0.25. The cell size for navigation maps can be changed by using the NavigationServer map_set_cell_size() function. The cell size for default navigation maps can also be changed in the ProjectSettings.

The instantiation of navigation_mesh in the constructor is not even needed, since that variable is only accessed through NavigationPolygon::get_navigation_mesh() where navigation_mesh is instantiated lazily, if necessary.

fixes #83570

[smix8]

Indeed, the navigation mesh init in the constructor is a leftover from the future (branch) that slipped in.

[akien-mga]

Thanks!