Make Optional<T> into readonly struct to avoid heap allocations

src/DotTiled.Tests/UnitTests/Serialization/MapReaderTests.cs

@@ -39,7 +39,7 @@ public partial class MapReaderTests
           return new Optional<ICustomTypeDefinition>(ctd);
         }
 
-        return Optional<ICustomTypeDefinition>.Empty;
+        return Optional.Empty;
       }
       using var mapReader = new MapReader(mapString, ResolveTileset, ResolveTemplate, ResolveCustomType);