Fix the handling of empty keys in PhutilJSONParser.

Summary:
Because the empty string is not a valid property of stdClass, the original JsonLint library replaced empty keys with '_empty_'. After D9623, all instances of stdClass have been replaced by arrays, so we can remove this nonsense.

Depends on D9623.

Test Plan: Added a test case and ran arc unit.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9633