Fix a warning in the JIRA field

Summary: Ref T3687. The `value` property may be `null`.

Test Plan: Loaded a revision with the JIRA field enabled but no issues attached, no longer saw a warning about a bad argument to `foreach()`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6890