*: check type based methods are not None

should be same for cases where type based method is a python script, but
is a bit more explicit/safe and consistent with other usages.
5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:53:43

failed ERP5.UnitTest-Master

01:22:27

failed ERP5.UnitTest-Master.Medusa

01:31:24

passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:00

passed SlapOS.Eggs.UnitTest-Master.Python3

00:22:17