[UsageConfing] config.usage.setupShowDefault bug

config.usage.setupShowDefault had a selection with an empty string
which causes "Really close without saving settings?" message when
exiting setup with no changes.

lib/python/Components/UsageConfig.py

@@ -138,7 +138,7 @@ def showsecondinfobarChanged(configElement):
 	config.usage.movieplayer_pvrstate = ConfigYesNo(default=True)
 
 	config.usage.setupShowDefault = ConfigSelection(default="spaces", choices=[
-		("", _("Don't show default")),
+		(None, _("Don't show default")),
 		("spaces", _("Show default after description")),
 		("newline", _("Show default on new line"))
 	])