NullReferenceException fix in TiledDocument.ReadXml

Fixes NullReferenceException when a map is loaded via a process with a native entry point; `Assembly.GetEntryAssembly()` can return null, and then the rest of it explodes. You can check this out by running TiledSharp via the XNA Content Pipeline or another application that doesn't have a standard `public static void Main` entry point, such as MSTest (I think).