Fix an issue where builds with no initiator failed to render in build plans
Summary: See PHI1743. If a build has no initiator PHID, the rendering pathway incorrectly tries to access a handle for it anyway.
- Set a build to have no initiator PHID.
- Viewed the build plan for the build.
- Before: fatal when trying to access the null handle.
- After: clean build plan rendering.
Differential Revision: https://secure.phabricator.com/D21269