Allow "Wait for Message" to be unset on build plans

Summary: Fixes T11910. I spent a couple of minutes looking for the root cause without much luck, but this will all be obsoleted by an eventual upgrade to EditEngine anyway.

Test Plan: Set and unset "Wait for Message", which now worked.

Reviewers: chad, avivey

Reviewed By: avivey

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11910

Differential Revision: https://secure.phabricator.com/D16919