Fix the handling of empty keys in `PhutilJSONParser`.

Summary:
Because the empty string is not a valid property of `stdClass`, the original [[https://github.com/Seldaek/jsonlint | 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 After D9623, all instances of `stdClass` have been replaced by arrays, so we can remove this nonsense.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