Add support for omitnil

[ivanjaros]

I have been fighting Go's json for years when it comes to handling of null values.
Go's implementation only has omitempty and it treats {}, [] and null as one and the same. But {} and [] are information of intent whereas null is not. I have been handling this issue in various ways and presently I am using a fork of native json library, after years of using Goccy and clean-up code that stripped null values from raw json. This was still more performant than using native json. Sadly the entire standard library lacks in performance, after a decade of neglect. But goccy has been lagging with issues and I no longer trust it, hence the fork.

In short, please add configuration option to not render null values. Also add omitnil or omitnull tag, so we can target maps and slices individually when we want to distinguish nil and empty values, since maps and slices are pointers in Go.

[zzhaolei]

I think this is related to golang/go#45669

[dsnet]

I suspect that your use-case is already satisfied by the omitzero option that that originated in this module and has since been back-ported to the "encoding/json" package for the upcoming release of Go.

[ivanjaros]

Technically it solves my issue but only on struct tag basis. I want a global configuration targeting only nils specifically, not all zero values.

[dsnet]

#61 adds a caller-specified option to omit all zero struct fields.

[ivanjaros]

global omitzero will get rid of false or 0, i don't know how else to communicate i want to get rid of null values in json, not default values in Go.

i'll stick with my forked native json because this brings no value to me. cheers.