resolves #669 Support blank versions
This implements blank i.e. version: '' support with a few simple tests.
I also set up a small example project https://gitlab.com/djencks/antora-blank-version-example to show this works on an actual site. I committed the built site to make it easier to see what happens.
The display of the "blank" version in the component explorer and page version explorer is questionable. Of course this is part of the UI, but perhaps this should be addressed now as well. In particular, with versions '' and '1.0', the page version selector won't let you go from '1.0' to ''.