Fix PHP 8.4 deprecation: Use explicit nullable type for substr $length parameter

Author: koriym

src/Rule/AbstractStrlen.php

@@ -138,7 +138,7 @@ protected function strlenIconv(string $str): int
      * @return string
      *
      */
-    protected function substr(string $str, int $start, int $length = null): string
+    protected function substr(string $str, int $start, ?int $length = null): string
     {
         if ($this->iconv()) {
             return $this->substrIconv($str, $start, $length);