I note from a discussion last year Compulsory Fields? that Null Fields weren't supported. Having just tried it again it seems this is still not supported. If this is the case, can anyone suggest a workaround for open text fields? I would imagine it is a pretty fundamental element to many workflows.
I'm not sure about the status of Collector's support for "not a null field" but aside from allowing null values, something to consider would be if you actually need the field in the service. If not, you can uncheck it as visible in the layer properties in Desktop before publishing. This allows you to remove the field form the service without actually having to delete it from the underlying table and the Apply Edits operation will then complete successfully.
In this instance I think we do need to view the fields requiring 'Not a Null Value' support, so this approach would be unsuitable. But this is a great bit of advice that I can implement on other projects so many thanks!