Return early if close button accepts input event

Summary:
Otherwise we close the effect whenever the close is pressed which is a
behavioural change.

That in turn leads to bigger bugs

BUG: 415155

Test Plan:
Ran effect
Clicked on the "Whitespace" of the dash
Clicked on the close

Now matches desktopgrid code

Reviewers: #kwin, zzag, ngraham

Reviewed By: #kwin, zzag, ngraham

Subscribers: ngraham, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27333