ComboBox: Arrays of strings don't have textRole.

Don't return a get function when the model is an array and
the return value is a string. So resolveTextValue() won't try
to retrieve a textRole in that case.

Change-Id: Ifc85eacc67accd34d63805487e20ae5829523c14
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>