ShopifyAuthorizationService authenticity checks throw exception when signature is null.

All auth service checks throw an exception when trying to call [`signature.ToUpper`](https://github.com/nozzlegear/ShopifySharp/blob/master/ShopifySharp/Services/Authorization/ShopifyAuthorizationService.cs#L56). A null signature means an invalid request, but the method should return false rather than throwing a null ref exception.
