New rule for explicit null values

[Sweetchuck]

There are three different ways to define a null value.

one:
two: ~
three: null

I think it would be useful to enforce the consistency.

[ota-meshi]

Thank you for posting this issue.
That rule of enforcing null notation sounds good to me.
However, I don't think we should use empty values, so I don't think the option to enforce an empty value is necessary.
By not enforcing empty values, we can avoid conflicts with rule that forbid empty values.
https://ota-meshi.github.io/eslint-plugin-yml/rules/no-empty-mapping-value.html