Incorrect null check in the constructor for ConnectorHealth and AbstractState

This bug is in relation to KIP-285: https://cwiki.apache.org/confluence/display/KAFKA/KIP-285%3A+Connect+Rest+Extension+Plugin

In the constructors of ConnectorHealth.java and AbstractState.java, the check that is done for the null parameters is done incorrectly. The current code only allows for the class to be instantiated if the parameters passed in are null. However the expected behavior has to be the opposite of this: we only want this class to be instantiated if the parameters passed in are not null. While the fix for this is pretty trivial, it would be good to add in some testing that tests the appropriate classes related to the rest extension.