MB-10540 Ignore empty server groups.

When checking that only one server group exists and converting such a
configuration into undefined. Because otherwise map_opts_hash would
change when an empty group is created. This happens because tags
property stops being undefined and becomes something like [{n1,
<<"0">>}, {n2, <<"0">>}]. Any additional number of empty server groups
doesn't lead to the same effect.

Initially my patch just removed this logic completely because it's not
needed anymore. But since every node decides *locally* if rebalance is
needed, this lead 2.5 nodes to believe that rebalance was always
required when there was only one server group.

Change-Id: I97965bec6d3c2487884093c1f1c13130f6d752a1
Reviewed-on: http://review.couchbase.org/34952
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>