REP-1048 Prevent a NPE if request-endpoint is not provided in the config

So... testing this is enormously painful because of the update listener and parent class. I think functional tests may be a better choice in this case? Those are pending.

I'll also note that I think it's silly to read the config object every time a handler is built when we're already reading it when we are notified that it has changed. The behavior is inconsistent. I'd probably refactor more of this class, but I want the PR to simply satisfy the acceptance criteria of REP-1048.