If there are no grouping field specified *but* group_by_no_leaf is
specified, should call read_group with no grouping fields: will
generate a single group (which can not be opened) for all of the
model.
Necessary for analysis views since individual "records" make no sense.
bzr revid: xmo@openerp.com-20130416092344-2pqog8f7xprn6hsh
Xavier Morel