code: public function getEmptyStringOrNull()
  	{
  		if (!isset($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']))
  		{
  			return '';
  		}
  
  		return static::getEmptyStringOrNullByFieldType($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['sql']);
  	}
docs: Return either an empty string or null based on the SQL string
  
  @return string|int|null The empty value