OR(FIND("SFR", ARRAYJOIN({which_network})), FIND("Orange", ARRAYJOIN({which_network})))
This will return 1 if either of the two terms appear in the multi-select field, and otherwise returns 0. This will let you duplicate the “has none of” behavior.
To set it up so that it will also return 1 if the field is empty, use:
IF({which_network}, OR(FIND("SFR", ARRAYJOIN({which_network})), FIND("Orange", ARRAYJOIN({which_network}))), 1)
That will allow you to filter to only get items that have some selection other than “Orange” or “SFR”. This is not quite the same as the “has none of” filter, but it might be useful for your purposes.