homed: don't insist on authentication against host-copy user record

homed maintains two or three copies of the user's identity record per
home directory: one on the host, one inside the LUKS header, and one
embedded in the home directory.

Previously we'd insist that if a user logs in they have to authenticate
against all three, as a safety feature. This broke logging into
unfixated records however, since in that case the host version is
synthetic and thus does not carry any authentication data.

Let's hence losen the strictness here: accept authentication against
host records that carry no auth data. This should be safe as we know
after all that the second/third record will catch invalid accesses.

Fixes: #15178