Fix PHP 8 deprecated warnings (#619)

This fixes issue #618

Author: matbech

src/JsonSchema/Constraints/UndefinedConstraint.php

@@ -60,7 +60,7 @@ public function check(&$value, $schema = null, JsonPointer $path = null, $i = nu
      * @param JsonPointer $path
      * @param string      $i
      */
-    public function validateTypes(&$value, $schema = null, JsonPointer $path, $i = null)
+    public function validateTypes(&$value, $schema, JsonPointer $path, $i = null)
     {
         // check array
         if ($this->getTypeCheck()->isArray($value)) {
@@ -111,7 +111,7 @@ public function validateTypes(&$value, $schema = null, JsonPointer $path, $i = n
      * @param JsonPointer $path
      * @param string      $i
      */
-    protected function validateCommonProperties(&$value, $schema = null, JsonPointer $path, $i = '')
+    protected function validateCommonProperties(&$value, $schema, JsonPointer $path, $i = '')
     {
         // if it extends another schema, it must pass that schema as well
         if (isset($schema->extends)) {