Adobe Flex SDK 3.0 (Release), Adobe Flex SDK 3.1 (Release), Adobe Flex SDK 3.2 (Release), Adobe Flex SDK 3.3 (Release), Adobe Flex SDK 3.4 (Release), Adobe Flex SDK 3.5 (Release), Adobe Flex SDK 3.6 (Release), Apache Flex 4.9.0
The following code returns an empty string:
var enc:XMLEncoder = new XMLEncoder();
However, SchemaMarshaller.marshall() correctly returns the string "null".
I believe the problems is that XML.appendChild (as used in XMLEncoder.setValue) for some reason treats the string "null" differently. I have no idea why.
If you're curious as to why I think this is a bug, see this StackOverflow question:
Finally, see this fiddle for examples: