More helpful exception

When calling config.get() with parameters that are null or undefined, you get an unhelpful exception thrown in Config.getImpl(): "TypeError: Cannot call method 'split' of undefined"

This PR fails earlier with a more helpful error message indicating the specific error the user made.
I could also resubmit with an explicit test for arrays and strings if preferred as those seem to be the only options getImpl can handle. I also did not update tests as there already is a test to ensure AN exception is thrown in the case of attempting to access an invalid property, however this PR updates that with a more helpful exception.