Fix warning when combobox has empty model.
Fix warning when combobox has empty model.

Change-Id: I676b75c984373d18439e57da9b31178fcea295b0
Reviewed-by: Gabriel de Dietrich <57b5c683f0b6abfc17ef151d5b68c8944258b63a@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,kobolabs/qtquickcontrols,richardmg/qtcomponents,kobolabs/qtquickcontrols,drgvond/qtquickcontrols,richardmg/qtcomponents,kobolabs/qtquickcontrols,mizukusak/qtquickcontrols,drgvond/qtquickcontrols,jfaust/qtquickcontrols,jfaust/qtquickcontrols,drgvond/qtquickcontrols,jfaust/qtquickcontrols,richardmg/qtcomponents