XMI12 — Minor bug / typo in handling of References

In the description of ReferenceAsElement in 9.5.5.1 on page 9-218, the
spec says:

"When the Reference's multiplicity has an upper bound greater than one
... a lower bound of zero, the reference is checked to see if any values
exist."

In reality, this check needs to happen >>always<<. Even in the [1..1] case,
the value may not exist if you are generating XML for an incomplete model.