Add missing nullability flag on CreateAccount return value

osu.Game/Online/API/IAPIProvider.cs

@@ -111,6 +111,6 @@ public interface IAPIProvider
         /// <param name="username">The username to create the account with.</param>
         /// <param name="password">The password to create the account with.</param>
         /// <returns>Any errors encoutnered during account creation.</returns>
-        RegistrationRequest.RegistrationRequestErrors CreateAccount(string email, string username, string password);
+        RegistrationRequest.RegistrationRequestErrors? CreateAccount(string email, string username, string password);
     }
 }