Don't process pad button events from a device that has been removed
This fixes a rare crash where we may dereference a null TabletV2Interface in findAndAdoptPad. This can happen because the underlying device disappears, and we call libinput_device_group_set_user_data (what tabletPadId.data points to) to reset the user data to null.
Similar to what we do to tablet tool events (see tabletToolEvent), we should check if this is null before continuing.
Fixes KWIN-3R6