Support for handling null when creating evalvalues

PorygonSharp/EvalValues/EvalValueCreator.cs

@@ -10,6 +10,8 @@ namespace PorygonSharp.EvalValues
     {
         public static EvalValue CreateValue(object o)
         {
+            if (o == null)
+                return new EvalValue(CreateNilEvalValue());
             var type     = o.GetType();
             if (type.IsEnum && !type.IsGenericParameter)
             {
@@ -64,6 +66,8 @@ namespace PorygonSharp.EvalValues
             return new EvalValue(ptr);
         }
         
+        [DllImport("libPorygonLang", EntryPoint = "CreateNilEvalValue",CallingConvention = CallingConvention.Cdecl)]
+        internal static extern IntPtr CreateNilEvalValue();
         [DllImport("libPorygonLang", EntryPoint = "CreateIntegerEvalValue",CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr CreateIntegerEvalValue(long l);
         [DllImport("libPorygonLang", EntryPoint = "CreateFloatEvalValue",CallingConvention = CallingConvention.Cdecl)]