Minor tweak to data-mw output for <ref name='foo' /> tags

* Serializer was complaining about missing source for these tags.
  Tweaked code to serialize these back to self-closed tag form.

* Tested with which rts this with the regular serializer in edit mode.
echo "one <ref name='foo'>bar</ref> two<ref name='foo' />\n<references/>" | node parse --extensions ref,references --wt2wt

Change-Id: Iaac63b26660d8b03af937d00d04540c3b8c0c86c