Currently, if debug-tweaks is not in IMAGE_FEATURES, we can't login as root, and there is no other users to login by default. Set a default passwd (such as root) can fix the problem.
I (and various others in the triage call) strongly believe we should not set a default root password. This could potentially be a huge security problem. As such we considered marking this bug as WONTFIX. I can see the case for improving the documentation about this and showing the user how they can set a default password should they wish to.
Please see RP's comments.