Clean up usage of m_client
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
Differential Revision: https://phabricator.kde.org/D23067