[FIX] orm: do not aggregate non-stored columns

Columns defined in the new api as interger, computed and non-stored should not be aggregated in read_group.
Fallback on False if column is None
Fixes odoo#3972, opw 619536

openerp/models.py

@@ -2109,7 +2109,7 @@ def read_group(self, cr, uid, domain, fields, groupby, offset=0, limit=None, con
             if f not in groupby_fields
             if f in self._fields
             if self._fields[f].type in ('integer', 'float')
-            if getattr(self._fields[f].base_field.column, '_classic_write')
+            if getattr(self._fields[f].base_field.column, '_classic_write', False)
         ]
 
         field_formatter = lambda f: (self._fields[f].group_operator or 'sum', self._inherits_join_calc(f, query), f)