[AppMenu] Ignore show request when application menu isn't configured

Before Plasma 5.12 one could only have either global menu or decoration button configured but since the setting was non-intuitive
it was switched to auto-enable when applet or button is present.
This results in both menus reacting to e.g. Alt+F for File menu.

BUG: 392012
FIXED-IN: 5.12.4

Differential Revision: https://phabricator.kde.org/D11580