Nullable type declaration for default null value (#328)

Adds ? before type declarations for parameters with a default null value

src/Validation/DNSCheckValidation.php

@@ -60,7 +60,7 @@ class DNSCheckValidation implements EmailValidation
      */
     private $dnsGetRecord;
 
-    public function __construct(DNSGetRecordWrapper $dnsGetRecord = null)
+    public function __construct(?DNSGetRecordWrapper $dnsGetRecord = null)
     {
         if (!function_exists('idn_to_ascii')) {
             throw new \LogicException(sprintf('The %s class requires the Intl extension.', __CLASS__));

src/Validation/Exception/EmptyValidationList.php

@@ -9,7 +9,7 @@ class EmptyValidationList extends \InvalidArgumentException
     /**
     * @param int $code
     */
-    public function __construct($code = 0, Exception $previous = null)
+    public function __construct($code = 0, ?Exception $previous = null)
     {
         parent::__construct("Empty validation list is not allowed", $code, $previous);
     }