Evdev keyboard handler emits keyevents with QString(0xffff)

Key events that have no text representation are by the evdevkeyboard
emitted as a QString(0xffff). Other keyboard event handlers appear to
all emit key events with null strings for these events.

Change-Id: If6b5c61a8cb76a6843238f834ce4feb4b73aa199
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>