yup.date().nullable() field does not result in nullable date prop in the JSON schema

[stijnjanmaat]

A schema like this:

const yupSchema = yup.object().shape({
  someDate: yup.date().nullable(),
  someString: yup.string().nullable(),
});

...results in a json schema like this:

{
  "type": "object",
  "default": {},
  "properties": {
    "someDate": {
      "type": "string",
      "format": "date-time"
    },
    "someString": {
      "type": [
        "string",
        "null"
      ]
    }
  }
}

I expect it to respect the nullable method on date(). So should result in:

{
  "type": "object",
  "default": {},
  "properties": {
    "someDate": {
      "type": [
        "string", 
        "null",
      ],
      "format": "date-time"
    },
    "someString": {
      "type": [
        "string",
        "null"
      ]
    }
  }
}

Could you add support for this?