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

src/applications/differential/harbormaster/DifferentialBuildableEngine.php

@@ -7,7 +7,11 @@ final class DifferentialBuildableEngine
     $object = $this->getObject();
 
     if ($object instanceof DifferentialDiff) {
-      return $object->getRevision();
+      if ($object->getRevisionID()) {
+        return $object->getRevision();
+      } else {
+        return null;
+      }
     }
 
     return $object;