code: 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']);
  	}
docs: Return the empty value based on the SQL string
  
  @return string|integer|null The empty value