You can cast Basri's Aegis without any target creatures. You'll just search for Basri, Devoted Paladin.

If do you choose targets and each target creature is an illegal target by the time Basri's Aegis tries to resolve, the spell won't resolve. You won't search for Basri. If just one target is still legal, however, you do put a +1/+1 counter on it and search for Basri.