ActionToolBar: Only show the overflow button if there are visible items in the menu

Summary:
This fixes https://bugs.kde.org/show_bug.cgi?id=415412 , which was occuring because the
overflow button did not properly check to see if the actions in the menu were actually
visible.

BUG: 415412

Test Plan: The example code from the linked bug report now works correctly.

Reviewers: #kirigami, ngraham, apol

Reviewed By: #kirigami, ngraham, apol

Subscribers: apol, ngraham, plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D26279