Add more explicit nullable types for default null values

Author: alexandre-daubois

Tests/Fixtures/MockStream/MockStream.php

@@ -28,7 +28,7 @@ class MockStream
      * @param string|null $opened_path If the path is opened successfully, and STREAM_USE_PATH is set in options,
      *                                 opened_path should be set to the full path of the file/resource that was actually opened
      */
-    public function stream_open(string $path, string $mode, int $options, string &$opened_path = null): bool
+    public function stream_open(string $path, string $mode, int $options, ?string &$opened_path = null): bool
     {
         return true;
     }