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/

editor/connections_dialog.cpp

@@ -664,7 +664,7 @@ void ConnectionsDock::update_tree() {
 				if (!(c.flags & CONNECT_PERSIST))
 					continue;
 
-				Node *target = c.target->cast_to<Node>();
+				Node *target = Object::cast_to<Node>(c.target);
 				if (!target)
 					continue;