It's not enforced, very often, but the rule on violent conduct includes an "attempt" and doesn't need to connect:
Violent conduct is when a player attempts to use excessive force or brutality against an opponent when not challenging for the ball, or against a team-mate, team official, match official, spectator or any other person, regardless of whether contact is made.
So by the strict letter of the law, if the ref thought that he tried (but failed) to headbutt Watt with excessive force, then he can give a straight red.