Morningtide: Rivals' Duel and Protection

[kirbysdl]

Hi, I hope this doesn't count as a question about cards from spoilers, as it came up in the Prerelease on Saturday.  My opponent plays Rivals' Duel on two of my creatures.  I play an Instant to give one of my creatures Protection from Red.  The head judge ruled that the spell would fizzle, which according to previous posts here is incorrect (one target is still legal, so the spell should resolve).  However, if the spell doesn't fizzle, how does the card's effect resolve?  It doesn't make sense for the remaining target to hit itself.  Does it simply swing at the empty air while not receiving any damage itself?  Or does something else happen?  Thanks!

[parallax]

If a target is illegal, the spell or ability can’t perform any actions on it or make the target perform any actions.

The spell will still resolve, but it won't do anything.

[Godder]

Parallax's post is confirmed by the FAQ, incidentally.