Client: Fix keyboard focus logic and crash

The previous solution did not check for nullptr before dereferencing, which
caused a crash. Furthermore, it checked the new ShellSurface's
shellManagesActiveState before deciding whether to unfocus the old one.

Task-number: QTBUG-55526
Change-Id: I410b6200a5b7b86806f70970730045a4a25f21db
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>