Don't generate CSGPolygon3D shape before the assigned path is inside tree

[MinusKube]

Fixes #71240

Within the NOTIFICATION_ENTER_TREE, we don't know at this point if the assigned path node has actually entered the tree. So I removed the _update_collision_faces() call here, and replaced it by a _make_dirty() call, like it's done in the set_use_collision method, so the update is deferred.

Also I added a small check inside the _build_brush() to return an empty brush if the path node is not inside tree.

[akien-mga]

Thanks!