Convert Object::cast_to() to the static version

Currently we rely on some undefined behavior when Object->cast_to() gets
called with a Null pointer. This used to work fine with GCC < 6 but
newer versions of GCC remove all codepaths in which the this pointer is
Null. However, the non-static cast_to() was supposed to be null safe.

This patch makes cast_to() Null safe and removes the now redundant Null
checks where they existed.

It is explained in this article: https://www.viva64.com/en/b/0226/

scene/3d/path.cpp

@@ -174,8 +174,7 @@ void PathFollow::_notification(int p_what) {
 			Node *parent = get_parent();
 			if (parent) {
 
-				path = parent->cast_to<Path>();
-				if (path) {
+				if ((path = Object::cast_to<Path>(parent))) {
 					_update_transform();
 				}
 			}