Fix issue where there's only one element on the menu
Summary:
This (random) height shows an odd padding on the menu when there's just the one
element.
CCBUG: 400517
Test Plan: I don't get it anymore on Discover
Reviewers: Frameworks, mart, ngraham
Reviewed By: mart, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D16685