Fix a NullPointerException when using OAuth with an empty subscription list

```java
Cannot get property 'items' on null object. Stacktrace follows:
java.lang.NullPointerException: Cannot get property 'items' on null object
        at org.seethrough.glass.MirrorService$$EOIJA6qu.listSubscriptions(MirrorService.groovy:159)
        at org.seethrough.glass.AuthorisationService.isUserSubscribed(AuthorisationService.groovy:200)
        at org.seethrough.glass.AuthorisationService.addSubscription(AuthorisationService.groovy:187)
        at org.seethrough.glass.AuthorisationService.bootstrapNewUser(AuthorisationService.groovy:156)
        at org.seethrough.glass.AuthController.callback(AuthController.groovy:31)
```