Improve rendering behavior for "foist upon" with an empty tokenizer

Summary:
Ref T13628. If you "Foist Upon", then delete the value in the tokenizer, then scroll down to the preview, you currently get an ugly "Unknown Object (???)" rendering.

This is technically correct in some vague sense and the transaction won't apply, but provide a more aesthetic rendering instead.

Test Plan:

Maniphest Tasks: T13628

Differential Revision: https://secure.phabricator.com/D21592