Summary:
This is to lift a unicode constraint to allow \u0000 (null) in the JSON string,
for backward data compatibility.

Test Plan: Jenkins

Reviewers: pengt

Reviewed By: pengt

Subscribers: webscalesql-eng

Differential Revision: https://reviews.facebook.net/D54543

(cherry picked from commit 01876b23180acd718a3ae4d11e59562c57a31827)