json: support @[json_null] tag to enforce null, when encoding none option values

[felipensp]

Implementing feature request.

_{Huly®: V_0.6-21751}

[spytheman]

Excellent work.

[enghitalo]

What about just @[nulltable] so this can make sense to others encoders as well

[enghitalo]

IMHO, @[required], or @[required; skip], or @[required; omitempty] should do the same work (What I think would be better)

[spytheman]

IMHO, @[required], or @[required; skip], or @[required; omitempty] should do the same work (What I think would be better)

No. The current name @[json_null] is better - it has json in it, and so it is restricted to only the json modules. This avoids the conflict with the decisions of other modules, that may also do serialization.

[spytheman]

omitempty in particular does the opposite, and is only valid for non option fields.

[JalonSolov]

Logically, @[omitempty] should apply to Option fields as well. If the Option is set to none (the default), then it is "empty". Same as a string field set to ''.

[spytheman]

The default for option fields, with a value of none, is that they are already omitted when serialized to json => adding a @[omitempty] tag to them is redundant to that default, and non effective for them.

[JalonSolov]

That sounds like "magic". It would make more sense to output null unless omitempty was used.

[spytheman]

No, it does not sound like "magic" - the default behavior is to not produce anything for option values that are none, and that saves resources, and that is not changed by the PR.