I was thinking about something like that too. But only very briefly.
I still believe accepting empty password is not by itself a problem. Real confusion is that fresh people are trying to enter password when there's no reason to enter password.
I'm specifically referring to fresh people because people like me or Trond are not fresh and are "tainted" by older experience where UI always demanded non-empty password even when target node did not have any. That was indeed stupid and very confusing. But that wrong behavior is gone.
So before doing anything I'd really like to understand why this confusion happens. And from Michael's words it appears that people are confusing our credentials fields as credentials to assign, not as credentials to try during join.
I want to make sure we're fixing real issue, not merely symptom (which again by itself I'm pretty sure is not at all a problem).