Improve behavior when there's no actions on the menu

Summary:
If there's no actions, we don't want to squeeze the component below
anymore.
Allows for UI's such as https://phabricator.kde.org/M91

Reviewers: Kirigami, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D4898