EZP-20867: implement FieldType::applyDefaultValidatorConfiguration()
Remove unused import
EZP-20867: hint specific field value, be specific about validation of empty value
EZP-20867: amend phpdoc with additional description
EZP-20867: add InvalidArgumentException to applyDefaultSettings()
EZP-20867: add tests for InvalidArgumentException
EZP-20867: enable loading empty array of options
EZP-20867: set default value or selectionRoot to null, enable empty selection root

Also refactored for readability
EZP-20867: enable empty selectionDefaultLocation, refactor for readability
EZP-20867: remove type check, applyDefaultSettings() is expected to be called before validation
EZP-20867: remove casting field settings and validator configuration to array