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