[client] Fix nullptr dereference in ConfinedPointer and LockedPointer

The setRegion call allows a null region. This means nullptr is an
allowed value which can be passed to ConfinedPointer::setRegion and
LockedPointer::setRegion.

In that case we crash if we try to convert the Region into a wl_region.
Thus add proper nullptr check, just like in
PointerConstraints::lockPointer and ::confinePointer.

Auto test adjusted to cover the condition.