[FIX] orm.read_group: check for empty groupby list

This is very ugly and needs a real refactoring,
just adding one more test for the time being.

bzr revid: odo@openerp.com-20111118153333-2skgbd197q8tb7uc

openerp/osv/orm.py

@@ -2516,7 +2516,7 @@ class BaseModel(object):
             d.update(alldata[d['id']])
             del d['id']
 
-        if groupby in self._group_by_full:
+        if groupby and groupby in self._group_by_full:
             data = self._read_group_fill_results(cr, uid, domain, groupby, groupby_list,
                                                  aggregated_fields, data, context=context)