[3.x] Fix NavigationObstacle not registering to default navigation map

[timothyqiu]

Fixes #64185

godot/scene/3d/navigation_obstacle.cpp

Lines 142 to 154 in faade5f

	void NavigationObstacle::set_navigation(Navigation *p_nav) {
	if (navigation == p_nav) {
	return; // Pointless
	}
	navigation = p_nav;
	if (navigation != nullptr) {
	NavigationServer::get_singleton()->agent_set_map(agent, navigation->get_rid());
	} else if (parent_spatial && parent_spatial->is_inside_tree()) {
	NavigationServer::get_singleton()->agent_set_map(agent, parent_spatial->get_world()->get_navigation_map());
	}
	}

Don't skip set_navigation() when navigation is null. The else if branch should be executed as it does not depend on navigation.

[akien-mga]

Thanks!

[timothyqiu]

Cherry-picked for 3.5.2