Support unobtainable creation dates (= None) in BTs
getCreationDate tries to obtain the date from the following sources (in that order):
the workflow history
Noneif it can't find any
The objects on which
getCreationDate is called (in this case):
do not have a workflow history;
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), this behavior will change and
None will be returned instead.