WD Comment: internal- and external-destination properties

Editors,

I found a possible nitpick between the internal-destination[1] and 
external-destination[2] property definitions in the 1.1 WD:

The acceptable and initial values defined for internal-destination are:

Value:     empty string | <idref>
Initial:     empty string

For external-destination, they are defined as

Value:     <uri-specification>
Initial:     empty string

Looking at the two, it appears that "empty string" should also be given 
as a value option for external destination.

Thanks,
Glen