Allow 'null' as assignee to set the issue to 'unassigned'.
PR: https://github.com/jenkinsci/jira-steps-plugin/pull/85
Fixed and merged to master, not yet released, pending release.
Released with 1.4.5, should make it to update center soon.
https://github.com/jenkinsci/jira-steps-plugin/releases/tag/jira-steps-1.4.5
PR: https://github.com/jenkinsci/jira-steps-plugin/pull/85