Drop checkArea helper

Summary:
So far some placement policies were handling special case when area is
a null rectangle. However, there is no good reason to do that. We can
just pass a valid area rectangle and remove pointless checkArea helper.
Doing so will make the code simpler and more comprehensible.

Reviewers: KWin, gladhorn

Reviewed By: KWin, gladhorn

Subscribers: gladhorn, romangg, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23923