Issue: empty strings as keys and keyRefs

[James Clark <jjc@jclark.com>]

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