erp5_test_result: handle cases where duration is None

We get a handful of useless messages in the logs otherwise:

TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

I feel like I never did so many MR on upstream in a week, it would be good if bugs would stop popping up.

This is draft because I know there are two other places to update, see https://lab.nexedi.com/tsoulard/erp5/-/commit/5e86291a69910da9a9920f8faff972e30939d5ad . Maybe should do a script at this point?

CC: @jerome