fix: check instance of storage using helper function

instanceof cannot be used to check the instance of a storage, doing so
breaks the check in certain cases. In this case, enabling the
`files_accesscontrol` app breaks the check.

Signed-off-by: Salvatore Martire <4652631+salmart-dev@users.noreply.github.com>

Author: salmart-dev

apps/dav/lib/Connector/Sabre/Directory.php

@@ -181,7 +181,7 @@ public function getChild($name, $info = null, ?IRequest $request = null, ?IL10N
 		// If we are, then only PUT and MKCOL are allowed (see plugin)
 		// so we are safe to return the directory without a risk of
 		// leaking files and folders structure.
-		if ($storage instanceof PublicShareWrapper) {
+		if ($storage?->instanceOfStorage(PublicShareWrapper::class)) {
 			$share = $storage->getShare();
 			$allowDirectory = ($share->getPermissions() & Constants::PERMISSION_READ) !== Constants::PERMISSION_READ;
 		}