In JSON DocumentEngine, preserve the distinction between "{}" and "[]"

Summary:
Ref T13635. Currently, the JSON DocumentEngine uses "phutil_json_decode()", but this can confuse "{}" and "[]".

Be more careful about how the JSON value is decoded, to preserve the distinction.

Test Plan:

Maniphest Tasks: T13635

Differential Revision: https://secure.phabricator.com/D21605