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.