MB-8961: Allow empty passwords

Change-Id: I5d48da4358e340a1e785b42a0b1249a803dc1b8c
Reviewed-on: http://review.couchbase.org/28564
Tested-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>