[FR]null value

[Gleb]

Is your feature request related to a problem? Please describe.
In order to delete a key in realtime db you can use either the delete method or send null as the value.
Sending null as a value can be beneficial sometimes because you can use the update method to delete several keys at one request.

Describe the solution you'd like
I would like a solution where None can be passed. Since requests lib does not want to change None values to null json.dumps can be used instead if None value is detected.

[Google Open Source Bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.