includeIfNull for @JsonSerializable in the configuration

[AntonShynkaretskyi]

Use case

By default all the models are generated without includeIfNull value.

Solutions considered

I haven't found any existing configuration for it. Json also doesn't seems to be supporting any kind of global config for it.

Proposal

It'd be awesome to be able to generate @JsonSerializable(includeIfNull: false) instead of @JsonSerializable() when necessary.