Void XML elements for booleans and null

[bpj]

Currently when converting YAML or JSON (I haven't (yet) checked with other formats) to XML (and back) booleans and null become strings "true", "false" and "null". I think it would be a better idea to represent them with void elements <true />, <false />, <null /> or perhaps better something like <constant value="true" /> (equivalent to the same as an empty rather than void element) etc. which would allow them to roundtrip properly.