No, there is nothing in the user experience that will test for null values, based on the field setting from the geodatabase that passed to the hosted feature service. However, as I understand it (I didn't test this), the database may reject creating a feature where fields have null values, but are set as not nullable. However, again, the user experience wouldn't trap any error and provide any user friendly interaction. My guess is (if this is in fact true) that the feature just wouldn't show up and the user would be left wondering what happened. This is just the state of where we are right now...
Thanks,
Mike