Fix unsafe uses of Callable.is_null()

`Callable.is_null()` is not equivalent to `!Callable.is_valid()` and
doesn't guarantee the call is valid.

editor/gui/editor_validation_panel.cpp

@@ -81,7 +81,7 @@ void EditorValidationPanel::set_update_callback(const Callable &p_callback) {
 }
 
 void EditorValidationPanel::update() {
-	ERR_FAIL_COND(update_callback.is_null());
+	ERR_FAIL_COND(!update_callback.is_valid());
 
 	if (pending_update) {
 		return;