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

src/applications/differential/field/specification/DifferentialJIRAIssuesFieldSpecification.php

@@ -145,6 +145,7 @@ final class DifferentialJIRAIssuesFieldSpecification
     $provider = PhabricatorAuthProviderOAuth1JIRA::getJIRAProvider();
 
     $refs = array();
+    if ($this->value) {
       foreach ($this->value as $jira_key) {
         $refs[] = id(new DoorkeeperObjectRef())
           ->setApplicationType(DoorkeeperBridgeJIRA::APPTYPE_JIRA)
@@ -152,6 +153,7 @@ final class DifferentialJIRAIssuesFieldSpecification
           ->setObjectType(DoorkeeperBridgeJIRA::OBJTYPE_ISSUE)
           ->setObjectID($jira_key);
       }
+    }
 
     return $refs;
   }