R1:a7bf279fd517

Don't try to publish build results to bare diffs

Summary:
See <https://discourse.phabricator-community.org/t/conduit-call-is-generating-phd-log-error-message/2380/>. If you run builds against a diff which is not attached to a revision (this is unusual) we still try to publish to the associated revision. This won't work since there is no associated revision.

Since bare diffs don't really have a timeline, just publish nowhere for now.

Test Plan:
- Created a diff.
- Did not attach it to a revision.