Sweeping (Attack Trait)
This attack has a wide sweeping arc. A Sweeping attack is one attack action, which comprises 2 attacks against different targets in the same zone. After the first attack, start a new attack sequence (beginning with the Declare Attack Step) targeting a different object in the same zone. Note: The second attack cannot be made against the same target as the first attack. If there is not a different valid target for the second attack, then the Sweeping trait has no effect this round.
When a Flying creature uses a Sweeping attack, if the first target has Flying, the second target must also have Flying. Similarly, if the first target does not have Flying, the second target must also not have Flying.
If a creature uses a Sweeping attack that also has Reach, then the above restriction does not apply; the second target can be Flying or non-Flying, independent of the first target.
The Underline and Bold are my doings....
"After the first attack, start a new attack sequence (beginning with the Declare Attack Step) targeting a different object in the same zone." This sentence says you have to declare a target after your first attack. This makes the act of declaring a target mandatory.
Your own creatures and mage are able to be targeted by an attack. So "If there is not a different valid target for the second attack, then the Sweeping trait has no effect this round." does not apply. This is due to your own controlled creatures and the mage itself in fact being a valid option for the attack.
I doubt this is intended by design but it is what I read.