Don't use isset in readFromJson
Don't use isset in readFromJson

Summary: `isset` isn't allowed in strict mode. This uses `idx` instead, which has the same behavior: the expression will evaluate to false if the key doesn't exist, or its value is null.

Reviewed By: yfeldblum

Differential Revision: D6027453

fbshipit-source-id: c8befe01ede7fc4a2ee6a43b6d554ea5c0bd75f1

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift