core-bundle/src/Resources/contao/library/Contao/Widget.php
public function getEmptyValue()
	{
		if (!isset($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']))
		{
			return '';
		}

		return static::getEmptyValueByFieldType($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']);
	}
public function getEmptyValue()
	{
		if (!isset($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']))
		{
			return '';
		}

		return static::getEmptyValueByFieldType($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']);
	}
Return the empty value based on the SQL string

@return string|integer|null The empty value