Nette\Forms\Controls\TextBase::getValue PHP Method

    public function getValue()
    {
        return $this->nullable && $this->value === '' ? NULL : $this->value;
    }

 /**
  * Rangle validator: is a control's value number in specified range?
  * @param  TextBase
  * @param  array  min and max value pair
  * @return bool
  */
 public static function validateRange(TextBase $control, $range)
 {
     return ($range[0] === NULL || $control->getValue() >= $range[0]) && ($range[1] === NULL || $control->getValue() <= $range[1]);
 }