[FIX] fields: safe check on ondelete cascade

In the case of attachment_ids, comodel._fields[inverse] can be res_id,
an integer field. Such fields do not have a default ondelete attribute.

opw 1945926

closes odoo/odoo#31864

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

odoo/fields.py

@@ -2438,7 +2438,7 @@ class One2many(_RelationalMulti):
                 vals_list.clear()
 
         def drop(lines):
-            if comodel._fields[inverse].ondelete == 'cascade':
+            if getattr(comodel._fields[inverse], 'ondelete', False) == 'cascade':
                 lines.unlink()
             else:
                 lines.write({inverse: False})
@@ -2484,7 +2484,7 @@ class One2many(_RelationalMulti):
                 vals_list.clear()
 
         def drop(lines):
-            if comodel._fields[inverse].ondelete == 'cascade':
+            if getattr(comodel._fields[inverse], 'ondelete', False) == 'cascade':
                 lines.unlink()
             else:
                 lines.write({inverse: False})