[Rfc-markdown] empty include files

[Michael Richardson <[e-mail]>]

While working on RFC8791 (sx:structure) updates to RFC8366bis, I would up
producing a zero length file which was then included. (Why the file is zero
is yet to be determined)

When this happens, I get:

xml2rfc -q -s 'Setting consensus="true" for IETF STD document' --rfc-base-url https://www.rfc-editor.org/rfc/ --id-base-url https://datatracker.ietf.org/doc/html/ --cache=/home/mcr/.cache/xml2rfc --css=lib/v3.css --metadata-js-url=/dev/null draft-richardson-anima-rfc8366bis.xml -o draft-richardson-anima-rfc8366bis.html --html
/corp/ietf/lwork/anima/voucher/draft-richardson-anima-rfc8366bis.xml(351): Error: Expected ascii-art artwork for <artwork type="">, but found b'<artwork xmlns:xi="http://www.w3.org/2001/XInclude" name="" type="" align="left" alt="" pn="section-5.1-2">\n</artwork>\n      '...

Line 351 says:
        <artwork name="" type="" align="left" alt=""><![CDATA[
]]></artwork>

As a result of:
~~~~
{::include yang/ietf-voucher-tree-latest.txt}
~~~~

Putting some content ("HELLO") into yang/ietf-voucher-tree-latest.txt makes
xml2rfc happy.

This feels like an XML2RFC bug in that it can't cope with an empty artwork.
The diagnostics from xml2rfc seems off too, since the input doesn't have what
was provided.

But, maybe kramdown-rfc2629 is at fault for daring to include no content.