Switch to is_string native method

Author: apetitpa

Constraints/UrlValidator.php

@@ -80,7 +80,7 @@ public function validate($value, Constraint $constraint)
         if ($constraint->checkDNS) {
             $host = parse_url($value, PHP_URL_HOST);
 
-            if ('' === $host || !checkdnsrr($host, 'ANY')) {
+            if (!is_string($host) || !checkdnsrr($host, 'ANY')) {
                 if ($this->context instanceof ExecutionContextInterface) {
                     $this->context->buildViolation($constraint->dnsMessage)
                        ->setParameter('{{ value }}', $this->formatValue($host))