Remove nullable from self:: to fix phpstan issue

Author: juniwalk

src/LocalesResolvers/Cookie.php

@@ -11,7 +11,7 @@ class Cookie implements ResolverInterface
 
 	public static string $parameter = 'locale';
 
-	public static ?string $expire = '+1 year';
+	public static string $expire = '+1 year';
 
 	private IRequest $httpRequest;