add test for empty case in JSON serializer

[ahmidbbc]

Signed-off-by: Abdelhamid Benmeziane [email protected]

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
b99561f

Please, read and sign the above mentioned agreement if you want to contribute to this project

[ezimuel]

Closing this since it's outdated.