feat: JSON emit_null option

[jackturnbull]

At the moment we're unable to emit null JSON (as this is the crystal default when an option is not supplied).

emit_null: if true, emits a null value for nilable properties (by default nulls are not emitted)
https://crystal-lang.org/api/0.30.1/JSON.html

We ought to be able to add an additional option which will then be forwarded to the options given to the JSON mapping for each property. I can't see a nice enough way to add this without an additional opt on the attribute definition, although this is where I'd expect it to be anyway.

Happy to submit a PR if you're okay with the design of this;

attribute description : String, emit_null: true

JSON.mapping(
  {% for name, opts in PERSIST %}
    {% if opts[:converter] %}
      {{name}}: { type: {{opts[:klass]}} | Nil, converter: {{opts[:converter]}}, emit_null: {{opts[:emit_null]}} },
    {% else %}
      {{name}}: { type: {{opts[:klass]}} | Nil, emit_null: {{opts[:emit_null]}} }
    {% end %}
  {% end %}
)

macro attribute(name, converter = nil, mass_assignment = true, persistence = true, emit_null = false, **tags, &block)
  # ...
end

[stakach]

totally missed this, yeah feel free to make a pull request