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.
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.

Commit also includes sorted imports and a few minor indentation fixes,
as well as another (very) minor string change.
Merge branch 'fix-refused-build-cli-npe' of github.com:marco-miller/jenkins into fork_fix-refused-build-cli-npe

Conflicts:
	core/src/main/java/hudson/cli/BuildCommand.java
Refactor string constant duplicate in BuildCommand and BuildCommandTest.
Merge branch 'fix-refused-build-cli-npe' of github.com:marco-miller/jenkins into marco-miller-fix-refused-build-cli-npe
Pull #979: noting.
[JENKINS-11347] [JENKINS-16692] might have been fixed by this, but unclear.