code: func (iter *Iterator) ReadNil() (ret bool) {
  	c := iter.nextToken()
  	if c == 'n' {
  		iter.skipThreeBytes('u', 'l', 'l') // null
  		return true
  	}
  	iter.unreadByte()
  	return false
  }
docs: // ReadNil reads a json object as nil and
  // returns whether it's a nil or not