If an empty password was supplied we ignored it and were trying to use
the fallback method to read the password instead (i.e. read from stdin).
However if that failed (which it always does if the cmp option -batch is
used) then we were reporting that we had successfully read the password
without actually setting one.

Instead, if an empty password is explicitly provided we should use it. If
no password is supplied explicitly and we have no fallback method then we
assume the empty password.

[extended tests]
Reviewed-by: NDavid von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12275)