Handle processId being 0 in ShellClient::killWindow

Summary:
If ClientConnection fails to get the process id (which happens at least
on FreeBSD) the process id is 0. We should not try to terminate this pid
as it terminates all processes is KWin's process group which is the whole
session.

Test Plan: Only compile test

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7168