Fix a phutil_json_decode call in CustomField PHID

Summary: I assume this is correct, it fixes my test case and was inconsistent with the rest of the file.

Test Plan: Add and Remove a CustomFieldPHID in Maniphest, no longer get errors.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8866

Differential Revision: https://secure.phabricator.com/D13642