Fix Herald exception when updating a diff that has carryover CCs

Summary:
This is pretty subtle and tricky, but some apply transcripts don't have a rule
ID because they're purely informational. We currently get an exception, which
prevnets diff updates.

jason/tuomas: don't update phabricator.fb.com until this lands :P

Test Plan:
Applied this patch live to secure.phabricator.com and was able to update D420.

Reviewed By: gc3
Reviewers: gc3, aran, jungejason, tuomaspelkonen
CC: aran, epriestley, gc3
Differential Revision: 421