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.

Test Plan:

• 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.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D21269