Make @select work with no document

[xatapult]

Sometimes you want to work with a sequence on a source port, regardless of whether there's actually a document coming in on it.

For instance, I just want to loop 10 times:

<p:for-each>
  <p:with-input select="1 to 10"/>
  ...
</p:for-each>

When in the example above the DRP of the for-each is empty, nothing will happen.

I would advocate for a change that makes this possible, regardless of whether there is something on the port.

[xatapult]

See #50 for a discussion about this.