[NbtkWidget] only fetch label of tooltip when it is set
[NbtkWidget] only fetch label of tooltip when it is set

Fixes bug 6019

jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx