Please add validation for `nullable:false`

[tigerinus]

Current validator respect required, but not nullable.

They are different. A field that is not nullable is not necessarily required. See https://swagger.io/docs/specification/data-models/data-types/#null

Here is an example:

    Mount:
      type: object
      properties:
        id:
          type: integer
          readOnly: true
        mount_point:
          type: string
          nullable: false
          example: "/DATA"
        fstype:
          type: string
          nullable: false
          example: "fuse.mergerfs"
        source:
          type: string
          nullable: false
          example: "/mnt/a:/mnt/b"
        options:
          type: string
          example: "defaults,allow_other,use_ino,category.create=mfs,moveonenospc=true,minfreespace=1M"
        persist:
          type: boolean
          description: |-
            `true` if the mount should be persisted in `/etc/fstab`, `false` otherwise
          default: false
        extended:
          type: object
          description: |-
            Extended properties of the mount
          additionalProperties:
            type: string
          example:
            "mergerfs.srcmounts": "/mnt/a:/mnt/b"

With this OpenAPI spec, validator would still be OK with it even I pass something like

{
    "mount_point": null
}