Hi Noogi,
Please see my answers to your question, hope it helps.
- What is the adobe campaign default in regards to null values?
Adobe Campaign default would use values that's set up within the data schema, if an attribute is specified as 'not null', it will be used as 'not null' and vice versa.
- If I choose "Empty values allowed", am I allowing values that are NULL and = '' or just one of these cases?
Yes, you are allowing both.
- If the above is just allowing NULL values, does the "Always populated" option consider a value = '' populated?
Always populated means it should contain a value for that field.
As an alternative to this you may also consider formatting the fields within the 'update' object. The advantage of this approach is that you can control the logic a bit more and need not change the settings everytime a new field is added to your file format. But yes, it's done using expression and may not be as intuitive as the wizard during the file format set up .
Have fun !
Thanks,
Madhan