[applets/appmenu] Show placeholder when configuring and empty

Summary:
Before this patch if you had no active menu and configured your plasmoid
it would be invisible.

After this patch a placeholder is shown in that configuration.

BUG: 403232

Test Plan:

• Window with menu - menu shown, no placeholder (as before)
• Window without menu - state is hidden so nothing is shown (as before)
• Window with menu whilst configuring - menu shown (as before)
• Window without menu whilst configuring - placeholder shown (new)
• Compact represenation remains the same

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18481