PaxosPrepare may add instances to the Electorate that are not in gossip

org.apache.cassandra.service.paxos.PaxosPrepare.RequestHandler#execute is given a list of electorate from the peer and attempts to compute its own, then replies back with this set.

On the peer side, we then have the set we sent and the set from the other instance... we then fetch the EndpointState from Gossiper and store into a Map, a map we later attempt to inject into Gossip.

It is possible that Gossiper does not know about the instance yet, so returns a null; causing a NullPointerException in downstream code.