Use SASL unless username is NULL, not password

This fixes not being able to connect to non-default buckets

Change-Id: Iac87f53075e0c9b07b79121462c136b1e0d8735a
Reviewed-on: http://review.couchbase.org/31221
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>