MB-11324: disable collect button if no selected nodes

Change-Id: I7462969e2d8280c0c4fc26760417c74920bc5c5c
Reviewed-on: http://review.couchbase.org/38008
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>