Verify that we have a value before using it

Contributes to CURA-446

cura/BuildVolume.py

@@ -257,7 +257,8 @@ class BuildVolume(SceneNode):
         if profile.getSettingValue("draft_shield_enabled"):
             skirt_size += profile.getSettingValue("draft_shield_dist")
 
-        skirt_size += profile.getSettingValue("xy_offset")
+        if profile.getSettingValue("xy_offset"):
+            skirt_size += profile.getSettingValue("xy_offset")
 
         return skirt_size