Add safety check in QWaylandInputDevice::Keyboard::modifiers()

::modifiers() might be invoked before being initialised. E.g. when
a pointer button event is emitted before the keymap is sent.

Change-Id: Ib95f9866d808b2a32ddfaa0862cdb3226fab1938
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>