add a check for uncomparable empty structs

[ybaldus]

#360, #362

[arp242]

Thanks; sorry it took me a while before I got around to reviewing this. I merged it via 8bbca55, but for some reason GitHub didn't recognize it 🤔

[ybaldus]

Thank you for merging it. Maybe because this was my first pull request? The tests didn’t run because the action would need a maintainer to accept it. Did you accept it?

[arp242]

I think it's because I did:

• gh pr checkout 366
• git rebase master
• (add a second commit with very small fixes)
• git checkout master
• git merge empty-structs

Usually git merge will spot that it belongs to a PR and will close it as merged. The problem here (I think) is that the rebase changed the commit hash from 58faf60 to 8bbca55, and it could no longer recognize it as belonging to this PR.