I feel strongly that we shouldn't introduce this NULLABLE_FIELD array. Just because we are stuck with INHERITABLE_METADATA doesn't mean that we should introduce another hard-coded list of fields with special behavior. 

This could be fixed either by changing the problem code so it doesn't rely on setting markdown to none, or "nullable" could be added as a property to ModelType in xblock. I do agree that None seems like a reasonable value for fields. @cpennington what do you think of this?