PromotionTypeHandler should check for ContentType before proceeding

Description

There are cases when customers add new promotion types, but they don't want to commit those to database yet. So, they set enableModelSyncCommit = false.

PromotionTypeHandler.SetPromotionsAvailability takes a list of Type and tries to set the availability for each of them, without checking for the present of the ContentType. A null check should be added.