Fix MimeTypeTest to prevent deprecations in PHP 8.1 and fix \Drupal\Core\StreamWrapper\PrivateStream::basePath() documentation

Hmmm... this is not going to work why this is causing a deprecation in PHP 8.1. Need to remove the change in #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves).

Oh i recorded this in the commit message...
ba0e1aad825 - Fix \Drupal\KernelTests\Core\File\MimeTypeTest (3 months ago) <Alex Pott>
:)

Trying a different fix on #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves) - see https://git.drupalcode.org/project/drupal/-/commit/114364910941ae69d58b1...

Yep we can fix \Drupal\KernelTests\Core\File\MimeTypeTest instead. Which is good because we rely on \Drupal\Core\StreamWrapper\PrivateStream::basePath() returning NULL in a few places... for example in \Drupal\Core\File\HtaccessWriter::defaultProtectedDirs:

  public function defaultProtectedDirs() {
    $protected_dirs[] = new ProtectedDirectory('Public files directory', 'public://');
    if (PrivateStream::basePath()) {
      $protected_dirs[] = new ProtectedDirectory('Private files directory', 'private://', TRUE);
    }
    $protected_dirs[] = new ProtectedDirectory('Temporary files directory', 'temporary://');
    return $protected_dirs;
  }

@andypost please read the previous comments and discussion. The fix in #11 was the fix on #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves) prior to discovering that the ONLY place this deprecation is triggered in core is while running the \Drupal\KernelTests\Core\File\MimeTypeTest. And that's because it incorrectly uses the private stream wrapper without the setting. Fixing like you have in #11 prevents anyone else from receiving the deprecation and then fixing it properly. Going back to #7 which has been proven to fix the deprecation on #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves) with the recent commits.

@andypost let's open a follow-up. I think getDirectoryPath() should throw an exception if ::basePath() is NULL. The change here is the minimum to get around the exception without changing any return values... ::getDirectoryPath() already returns a NULL in this situation. That's what is happening in HEAD, it's not right and we can improve but it's not necessary to fix the deprecation.

See \Drupal\Core\CoreServiceProvider::register() - we only register the wrapper if this setting is set...

    // Only register the private file stream wrapper if a file path has been set.
    if (Settings::get('file_private_path')) {
      $container->register('stream_wrapper.private', 'Drupal\Core\StreamWrapper\PrivateStream')
        ->addTag('stream_wrapper', ['scheme' => 'private']);
    }

Hmmm actually #17 points to an even better fix. It turns out that in kernel tests the private stream wrapper is being registered by \Drupal\KernelTests\Core\File\FileTestBase::register() - so that test base should properly create a private files directory so that it works as expected.

@andypost thanks for creating the follow-up. Just realised I hadn't asked the question as to what was registering the private stream wrapper - turns out it is \Drupal\KernelTests\Core\File\FileTestBase - so that's where we need to set the setting.