Replace empty user with dash in access.log.

Change-Id: I8d7863f27a31ea3a89ae7ef46db87d7a38bcde3d
Reviewed-on: http://review.couchbase.org/37940
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>