x11client: Prevent trying to place clients to invalid areas (aka when no output is ready)

It is to prevent crashes on launch such as:

(this=this@entry=0x7fff2de4b580, msg=msg@entry=0x7fe3cf672ea0 "ASSERT: \"%s\" in file %s, line %d") at global/qlogging.cpp:893
at ../../include/QtCore/../../src/corelib/global/qlogging.h:90
(this=0x55ab69121cd0, c=0x55ab69225190, area=...) at /home/mca/kde/src/kwin/src/placement.cpp:459
(this=0x55ab69121cd0, c=0x55ab69225190, area=..., policy=KWin::Placement::Centered, nextPlacement=KWin::Placement::Unknown)
at /home/mca/kde/src/kwin/src/placement.cpp:83
at /home/mca/kde/src/kwin/src/placement.cpp:67
at /home/mca/kde/src/kwin/src/x11client.cpp:666
at /home/mca/kde/src/kwin/src/workspace.cpp:682
at /home/mca/kde/src/kwin/src/events.cpp:221
at /home/mca/kde/src/kwin/src/main.cpp:477
(this=0x55ab6874a740, eventType=..., message=0x55ab69275ed0, result=0x7fff2de4be40) at /home/mca/kde/src/kwin/src/main.cpp:561
(this=<optimized out>, eventType=..., message=0x55ab69275ed0, result=0x7fff2de4be40) at kernel/qabstracteventdispatcher.cpp:495
(f=(void (KWin::Xwl::Xwayland::*)(class KWin::Xwl::Xwayland * const)) 0x55ab66df723a <KWin::Xwl::Xwayland::dispatchEvents()>, o=0x55ab6909be60, arg=0x7fff2de4c050) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:152

cc @vlad @davidedmundson