OF-1826: Prevent NPE in new DataForms implementation.
The implementation introduced by OF-1809 causes collections of DataForm instances to be processed.
The new contract defines that a set must be present - null values are not allowed.
It's easy to overlook this new contract. For example, our own code returned a null value, instead of an empty collection.
This commit modifies the return of a null value (it now returns an empty value). Additionally, this PR adds a guard for problems like this, by explicitly checking of a value is null.
