Json Stringify option to not write out null optional fields

[hazeycode]

Fixed version of #8908 with added test to parse stringify output

[hazeycode]

Found another failure case. Needs more testing.

[daurnimator]

I wonder if a better option might be a skip_defaults to avoid rendering all defaults.
Then you could opt-in per optional field by setting their default value to null.

[hazeycode]

I wonder if a better option might be a skip_defaults to avoid rendering all defaults.
Then you could opt-in per optional field by setting their default value to null.

I'm unconvinced. It does make sense but is there a use-case where you don't want to stringify nulls that go OTW but would want to stringify some local default that doesn't make sense as a default on the other end? i.e.

const Message = struct {
    user_id: u32 = 1337, // should stringify
    optional: []const u8 = null, // should not stringify 
    required: []const u8, // should stringify
}

Of course, the problem here is easily avoided, however, not convinced that it's a good user experience.

[hazeycode]

This appears to be working properly now!
Commits likely want squashing but I had all kinds of bother trying to rebase

[andrewrk]

Looks redundant with field_output