MDL-79384 core: fix modal footer logic when rendering it from template

If the footer property is a Templates.render() promise, and the modal is
set to show immediately (show=true), the show() logic - which contains
the footer show/hide logic - can run before the footer content is set
(i.e. before the promise chain is complete). In such cases, the footer
will be marked as hidden and must be made visible again.

lib/amd/src/modal.js

@@ -642,6 +642,7 @@ export default class Modal {
             })
             .then((footer) => {
                 this.footerPromise.resolve(footer);
+                this.showFooter();
                 return;
             })
             .catch(Notification.exception);