Fix return value of "image set" command

"image set" command should return None. But in one path,
it returns ({}, {}). This patch fixes this.

Change-Id: I3847e661cb7e89863921a3f0a859d9b1a8077ede

openstackclient/image/v1/image.py

@@ -691,7 +691,7 @@ class SetImage(command.Command):
 
             if not kwargs:
                 self.log.warning('no arguments specified')
-                return {}, {}
+                return
 
             image = image_client.images.update(image.id, **kwargs)
         finally: