Don't translate a null rect.

This was probably because of some forgotten braces, since the
indentation seems to indicate that.
The current behavior might cause a null rect to become not null, which
might cause for QRect::isNull() to give a false positive.
Worst case it might case a program to crash (i.e. div by 0)

Change-Id: Iba49699880250e605b427ff4c1228c68a3471a2f
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>