If the creature you control is an illegal target as Joust tries to resolve, no creature gets +2/+1. If that creature is a legal target but the creature you don't control isn't, it still gets +2/+1 until end of turn if it's a Knight.

You can't cast Joust unless you choose both a creature you control and a creature you don't control as targets.

The target creature you control gets +2/+1 only if it's a Knight as Joust resolves. It'll still fight if it doesn't get +2/+1.

If either target is an illegal target as Joust tries to resolve, neither creature will deal or be dealt damage.