Allow NullFlavor system for DocumentReference.type (v4 allows UNK)
Correct a couple v4 typos in Procedure.code systems (later versions have corrected URLs and we use those instead of the v4 URLs)
If a field does not exist, don't flag the row - that's more of a profile compliance thing, and might even be valid (in cases like Vital Signs that don't use Observation.valueCodeableConcept)
This is a companion PR to similar qualifier spec changes.