Summary:
Fixes T12082. Ref T11114. When modular transaction render a handle list, they use HandleListView, which has a text mode.

However, the HandleListView is a TagView, and currently TagViews always render a tag of some kind. Allow them to return `null` to decline to render any tag.

Test Plan:
  - Added a pile of debugging stuff to `ApplicationTransactionEditor` to throw during mail generation.
  - Added a reviewer to a revision.
  - Used `bin/worker execute --id ...` to hit the mail generation repeatedly.
  - Before patch: mail generated with a <span>, even in text mode.
  - After patch: clean mail generation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12082, T11114

Differential Revision: https://secure.phabricator.com/D17162