fix NPE when launching downstream build using promotion action

I set up a promotion that contained an action to trigger a downstream build. Upon running the promotion I received 
```
java.lang.NullPointerException
	at org.jvnet.hudson.plugins.DownstreamBuildViewUpdateListener.onStarted(DownstreamBuildViewUpdateListener.java:85)
	at org.jvnet.hudson.plugins.DownstreamBuildViewUpdateListener.onStarted(DownstreamBuildViewUpdateListener.java:46)
	at hudson.model.listeners.RunListener.fireStarted(RunListener.java:188)
	at hudson.model.Run.run(Run.java:1429)
	at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:239)
```
in the console output for the triggered job, which failed the build immediately. This pull request is a simple null check to avoid this from happening when a non-project triggers a downstream job.