Stabilize explicitNulls in Json

[qwwdfsad]

It is here from 1.3, we get the overall positive feedback and there is not many reasons to keep it experimental any longer

[sandwwraith]

Apparently it depends on #2586 now, so it makes sense to think about them together

[sandwwraith]

Other issues that may be related to explicitNulls: #2422, #1542. But that seem to be all of them, there isn't a lot going with this feature in the tracker.