should reject an scxml document containing data without @id

[mattoshry]

Per [1], data/@id is required.

SCION should reject the following:

<scxml xmlns="http://www.w3.org/2005/07/scxml"
  version="1.0">

<datamodel>
  <!-- missing id -->
  <data/>
</datamodel>

<state id="uber">
  <transition event="*" target="fail">
    <log expr="'unhandled event ' + JSON.stringify(_event)"/>
  </transition>
</state>

<final id="fail"/>

</scxml>```

[1] https://www.w3.org/TR/scxml/#data