"Sign with MFA" action on differential revisions creates strange feed entries


Observed Behavior:
When you comment on a Differential Revision with the “Sign with MFA” action, it creates a Feed entry with no summary. Attached are screenshots of the Activity dashboard widget, the feed popup, and the Feed application:


This also crashes the feed HTTP hook service we’re using, although that’s our problem, not yours.

Expected Behavior:
Posting a message like “jbrown commented on D1234567 and signed it with MFA”

Phabricator Version:
Local fork based on 6bca2e077330af12a5551d4eda63a6e239d89e9f (2019 Week 4)

Reproduction Steps:

  1. Set up MFA on a user
  2. Create a Revision
  3. Add a comment with “Sign with MFA” as the action from the dropdown


Thanks, see https://secure.phabricator.com/D20071.