Fix undefined check in global drawer menu mode

Summary:
Kamoso uses Overlaydrawer instead of GlobalDrawer, which has no isMenu property

BUG: 417956

Test Plan: Kamoso doesn't have an empty second hamburger button any more

Reviewers: Kirigami, mart, ngraham

Reviewed By: Kirigami, ngraham

Subscribers: apol, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27996