MB-11647: avoid mass_xxx_docs_estimate calls with empty vbuckets

This is important when there's mixed version cluster that does not
support upr estimates. And it may help for performance in expected
case of empty tap-replicated vbuckets.

Change-Id: I09a938ec0079f41a39c3904a44e21c0c1482e4aa
Reviewed-on: http://review.couchbase.org/39164
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>