Allow source to be empty on p:invisible-xml

[ndw]

It's uncommon, but perfectly legitimate, to have an Invisible XML grammar that matches the empty string.

But it's slightly challenging to turn "nothing was selected" (an empty sequence) into a text document that contains only the empty string.

I think we should allow a sequence on the source port with the semantics that an empty sequence means the grammar parses an empty string, a single input works as it does today, and more than one input is an error.