there have not been any changes to Not a Null value. When you create a service with allow null values = false the template generate has " " (space) or 0 inserted into the template as default values during the publishing of the layer. Adding required field support for collector is coming to the platform but is not included into the recent update 10.3.1
Thanks for the template clue Russell. I've realised I can stop Collector starting with a space for a non-nullable text field by changing the template in AGOL - e.g "FIELD" : " " becomes "FIELD" : "" - and this gives me the behaviour I want, users have to input something! Do you know of any problems with this approach?