When I add a blank line in Census Assistant for (say) a lodger’s details (or someone not in the tree) I get Error 20 Invalid GEDCOM format when I come to save. The data is written to DB and is shown OK when I reload the page. I remember this from an earlier version of webtrees several months ago.
Not just several months. That bug has been around for a couple of years. As you say, it doesn’t actually effect the operation at all, so it’s not been a priority to fix. I’ll get around to it eventually I hope.