Null check for if build scheduling Refused by extension => not in Queue.

No test added in hudson/cli/BuildCommandTest.groovy, as this case is no
longer reproducible with bare Jenkins core and cli. Indeed, one now
need a plugin to extend QueueDecisionHandler, so it can Refuse a build,
hence test this specific null case. Regression tests do all pass still.

Thx to Scott's preceding pull-req for the initial idea: https://github.com/jenkinsci/jenkins/pull/955
-As agreed with Scott, I did resume the work on it (hereby).

Related issues, which I was unable to reproduce with latest master code:
https://issues.jenkins-ci.org/browse/JENKINS-16692
https://issues.jenkins-ci.org/browse/JENKINS-11347