Unbreak milestone creation over Conduit API (project.edit).

See also:
https://github.com/grembo/phabricator/commit/0851b89eb6633dd792cd4eb10c26f86c2f0da56a
https://discourse.phabricator-community.org/t/call-to-a-member-function-getphid-on-a-non-object-while-creating-milestone-using-conduit/3370/4

Reported by: Fabian Abplanalp - Legatech GmbH <fabian@legatech.ch>