[JSON] fix bug that incorrectly allowed trailing commas after an empty container

[markdroth]

No description provided.

[rockspore]

I sort of got how the added line would help the parser return error on the } or ] right after that redundant ,. But it'd take me some more time to understand why it has always worked when the stack is not empty before that ,.

[markdroth]

I believe the intent here is that container_just_begun_ indicates when the previous character is a { or [. We're using that to determine if it's okay to accept a } character when we're expecting an object key or a ] character when we're expecting an array element. However, the only place that we were setting it back to false was here, and that code-path was not used when we saw a } or ] character, so we were never resetting it to false in the case that the container was empty.