KCM KWinScreenEdge fix build issue

Summary: Remove Q_ASSERT() and add a None value to Monitor::Edge enum to convert ElectricBorder NONE or COUNT

Test Plan: Fix kwin build

Reviewers: KWin, ngraham, zzag

Reviewed By: KWin, ngraham, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29104