Fixes incorrect type narrowing of nullable string

[lee-hyunkyu]

When creating a Joi schema with modifiers .allow(null).required(), the type of the resulting object is not correctly nullable even if it passes the runtime check

const schema = joi.object({
   x: joi.string().allow(null).required()
})

const result = joi.attempt({ x: null }, schema)
// expected: typeof result = { x: string | null } 

Current Behavior

Expected Behavior