Check for existence of webhook url

This fixes a crash if you have services installed.
We, for example, have a Jenkins installation hooked up to our GH:E. Jenkins is integrated as a "service" into the hooks and returns the following information as webhook.config:

```{ jenkins_hook_url: 'http://ci.secret.host/github-webhook/' }```

This lead to an error of ```TypeError: Cannot call method 'indexOf' of undefined```.