Don't show "Start pipeline from here" in pipelines where no event exists

[wahapo]

What happened: When an event does not exist yet, click Start pipeline from here to start the build, the build will not start with Loading displayed.

What you expected to happen: In this case, I don't want to show Start pipeline from here. Or, if the event does not exist, don't show the graph even if turn on Show triggers.

How to reproduce it:

1. Prepare a pipeline where no events exist. (ex. https://cd.screwdriver.cd/pipelines/6420/events)
2. Turn on Show triggers, and click Start pipeline from here for any job.
   
3. Click Yes.
   
4. Build will not start with Loading displayed.
   

[tkyi]

Agree, shouldn't show or give options in the UI for actions if they are not permitted or possible.