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.

• 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.