XMI12 — How to represent a "null" for a Class-valued Attribute?

Suppose that my meta-model has a Class whose Attribute has another Class
as its value. How do I represent this attribute in an XMI document when
its value is a null object reference?

According 9.5.4.2, the document should contain something like this:

<pkg.cls.attr>
<pkg.othercls xmi.idref="" />
</pkg.cls.attr>

Unfortunately, this doesn't work as the empty string is not legal as an
IDREF value.

As far as I can tell the 9.3 EBNF rules (e.g. production 7h) don't even
consider the case where an Attribute's type is a Class.