Unserialization exception on empty JSON body (only in PHP7)

Since PHP7, json_decode throws an syntax-error exception when the data is empty.
When given a empty object, it will work correctly ( '{}' instead of '""' )

When we call a service REST endpoint, which return a 200 http response with an empty body, we got an exception.

I've created a little patch to resolve this when the endpoint return an empty body instead of an empty object.