Clean up usage of m_client

Summary:
We have a smart pointer, so be consistent in checking it.
Since there is an operator->, make the code easier to read by removing countless .data()->.
!m_client.isNull() can also be written as m_client since operator bool works.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23067