Save to JSON should use null, not encode <nil> as a string

[artfulrobot]

The JSON save method encodes NULL sql values as "\u003cnil\u003e". It should simply output null.

[kndndrj]

Agree!

It just isnt a prio atm.

If you wish, you can explore a bit around this in dbee/outputs/json.go

[artfulrobot]

Never written a line of Go before, so I'll leave that as someone else's good first issue! Thanks.