Support unobtainable creation dates (= None) in BTs

In general, getCreationDate tries to obtain the date from the following sources (in that order):

1. the workflow history

2. the creation_date attribute

3. returns None if it can't find any

However:

The objects on which getCreationDate is called (in this case):

• do not have a workflow history;

• nor the creation_date attribute.

The date is actually obtained from the portal ("portal.creation_date") by Acquisition.

Because of an incoming change related to CMF (we drop Products.CMFDefault)(see !957 (merged)), this behavior will change and None will be returned instead.