Always emit Client::captionChanged when the caption changed

Summary:
With ddf3536f19a4243670e5fc87f5fbaf9e44031542 a regression got
introduced resulting in Client::captionChanged not being emitted if
Client::setCaption was called without the force parameter.

The regression is only in master branch.

BUG: 383444

Test Plan: Added test case passes

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7290