My guess is you selected the disclaimer displayed in the tooltip about unsecure logins, so the login field was empty.
I'm pretty sure I didn't, I always look at the dropdown list before hitting Return, since on some pages where it happens I have more than one login so I need to make sure I pick the correct one. And like I've said, I'm relatively sure I've seen it fill out the username field (i.e. the username is shown in the input field before submission). It mostly happens when FF is running slow, i.e. when you can see e.g. the password manager dropdown being drawn step by step, and after hitting enter there's a pause of half a second or so before it actually submits the form
P.S.: Also, if the problem was that I did select the disclaimer, why is it selectable in the first place? Wouldn't it be better to either make the keyboard navigation skip over it or at least ignore selections?
P.P.S.: I've just had it happen on a https site (valid certificate from LetsEncrypt), so the unsafe warning doesn't seem to be a factor in this