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.
• Created a build plan with "make http request + wait for response".
• Manually ran the build plan against the bare diff.
• Used bin/phd debug task to run the build and hit a "revision not attached" exception during publishing.
• Applied patch.
• Ran bin/phd debug task, got clean (no-op) publish.
• Sent build a failure message with "harbormaster.sendmessage", got a failed build.

This isn't a real workflow, but shouldn't fail.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20156