Drop nil voters when handling misbehaviour

[sainoe]

Problem

Validators who vote nil don't have their signature verified in comet. Consequently, they should not be extracted as byzantine validators when handling MsgConsumerMisbehaviour, find the complete bug report here.

Edit: It's also critical that we check that Misbehaviours comes from a valid consumer chain light client. Otherwise the above isn't sufficient to stop an attacker from introducing bogus validator signatures.

Props to @insumity for finding this issue ❤️.