Você não precisa mirar em outra criatura (RAI)
O cantrip Green-Flame Blade (do SCAG) tem uma redação semelhante:
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails. On a hit, the target suffers the attack's normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.
Jeremy Crawford, principal designer de regras, disse o seguinte no Twitter :
What does the leap effect of green-flame blade do if there are no hostile targets nearby? Does it jump to allies?
The intent is that you can choose no one. If you can't see, you can't choose anyway, and the flame halts
Dado o texto similar ao Chaos Bolt, parece seguro assumir que o Chaos Bolt funcionaria da mesma maneira, então se não houver inimigos dentro desse intervalo de Chaos Bolt, você não precisa mirar em outra pessoa.