Don't kill server if web hook domain is null
Don't kill server if web hook domain is null

The server dies when the web hook is not a valid web hook. For now I’ve
resorted to testing `webhook.domain` with null, but not sure if that ’s
the right approach. This keeps the server running even if a unexpected
web hook is received.

probot/probot,probot/probot,bkeepers/PRobot,pholleran-org/probot,pholleran-org/probot,bkeepers/PRobot,pholleran-org/probot,probot/probot