Fix exception when trying to save global setting mail tags

Summary: Ref T11098. Template preferences don't have a user, but this codepath didn't get fully updated to account for that.

Test Plan: Saved mail tags in global prefernces.

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T11098

Differential Revision: https://secure.phabricator.com/D16050