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

New test in hudson/cli/BuildCommandTest.groovy, as well as companion
@TestExtension class, thanks to Jesse' suggestion. This is so that we
test this hereby fixed case of a QueueDecisionHandler Refusing a build,
leading to that null value now checked and handled accordingly.

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