MB-8778: Allow zero views

The couch_view_file_sorter allows to have zero views.
That is needed for the incremental update of spatial
views, where the file sorter is only used fro the
id-btree, but not for the views themselves. The views
are updated within the Erlang space.

Change-Id: I5d1650d1479fa877e7f72e1970cdb111aedd8d36
Reviewed-on: http://review.couchbase.org/28055
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>