Fix Test and Test Coverage

Some Message classes are prone to nullpointer exceptions

eg:

public boolean equals(Object o) {
        if (o instanceof HandoffCompletedMessage) {
            HandoffCompletedMessage m2 = (HandoffCompletedMessage) o;
            return akkaSystemId.equals(m2.akkaSystemId); //akkaSystemId can be null here
            // probably better;
            // return Objects.equals(akkaSystemId, m2.akkaSystemId); // does a nullcheck
        }
        return false;
    }