Using empty string as default terminal avoids storing a path to
QtCreator resources (which becomes invalid if creator is deleted)
on mac.

Change-Id: I9d5fe9bce38387d82de451652df6be9ec5eb5960
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>