modalDialog: Consistently return correct boolean for open() in ModalDialogs

Previously these calls either ignored the return from super.open() or
implicitely returned false.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2038>

js/ui/welcomeDialog.js

@@ -26,9 +26,9 @@ class WelcomeDialog extends ModalDialog.ModalDialog {
 
     open() {
         if (!this._tourAppInfo)
-            return;
+            return false;
 
-        super.open();
+        return super.open();
     }
 
     _buildLayout() {