When a null C# string is passed to UTF8_ToNative(), return a null pointer instead of an empty C string.

src/SDL2.cs

@@ -44,10 +44,17 @@ namespace SDL2
 		#region UTF8 Marshaling
 
 		internal static byte[] UTF8_ToNative(string s)
+		{
+			if (s != null)
 			{
 				// Add a null terminator. That's kind of it... :/
 				return System.Text.Encoding.UTF8.GetBytes(s + '\0');
 			}
+			else
+			{
+				return null;
+			}
+		}
 
 		internal static unsafe string UTF8_ToManaged(IntPtr s, bool freePtr = false)
 		{