use [] and not undefined for missing vclock in search_with_vclock

Because [] is valid vclock and undefined is not. Particularly, we can
pass [] to vclock:count_changes

Change-Id: I35cdc01096dd96b6559f780dc0a9f6f0a4a8006f
Reviewed-on: http://review.couchbase.org/44451
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>