[JENKINS-17728] Fixing another possible cause of an NPE.

Unlike the bug I originally reproduced, in which the parent had some builds
but the context in which the new build was scheduled omits a parent,
this seems to be due to a case in which there is no build record at all for the parent.
No idea how that could happen (getLastBuild should return even a running, failed, or aborted build),
but @treydock reports a stack trace in 1.509.2 which implies this.
So fixing null safety; will not prevent an exception but will report it more gracefully.