Issue: empty strings as keys and keyRefs

We need to think about what happens if an element with key or keyRef 
content has an empty string.  For example, if the schema has:

<element name="foo">
  <choice>
    <empty/>
    <key name="bar">
      <data type="string"/>
    </key>
  </choice>
</element>

and the instance has

  <foo></foo>

then it is ambiguous what the key generated by the match is. One the other 
hand I don't think our ambiguity algorithm handles this.

I think the simplest solution will be to say that <key> and <keyRef> don't 
match empty strings even if the included pattern can match an empty string, 
eg

<element name="foo">
  <key name="bar">
    <data type="string"/>
  </key>
</element>

does not match

<foo></foo>

James