jjb: Immediately remove currentJobs with null build

jjb: Immediately remove currentJobs with null build

In 9ee19c2b8f8af895460c3cc132282a9f85ec27dd, jobs ended up getting set
as 'Cancelled' if LaunchJob returned null instead of a Job
object. Those cancellations meant that the entire result of the
trigger was set to 'Aborted', masking otherwise successful runs.

This commit changes the behaviour slightly by prunning entries with a
null build immediately after the jobs are queued. This means that jobs
that couldn't be found by name don't cause the build to be flagged as
'Aborted'.

Change-Id: Ie76bb7bf05625eb541b0abdf0f05e95eb2d740c4