Fix service UI

There was a small bug in the service UI that caused an empty 'Trigger' label to be shown. After this change the 'Trigger' field is not shown when only one event type of supported.

Before:
![Screen Shot 2015-03-16 at 7.40.31 PM](https://gitlab.com/uploads/dblessing/gitlab-ce/0457a0b918/Screen_Shot_2015-03-16_at_7.40.31_PM.png)

I think in the future there are other things we can do to enforce at least one selected event type and also show the user what the singular event type is if only one is supported. That will take time to work out and this is definitely acceptable for the time.

See merge request !395