I'm not sure if I've missed some subtlety.. but in the description of resource representations does Occurs 0 mean the field is optional, or does it mean the field must be there but could be empty?

If the former would it not be useful to have an optional field, say required, which lists the fields required for the representation to be valid - useful for paranoid clients/debugging, clients dealing with different spec versions and different vendor implementations.
If the required fields are stated explicitly then the spec could tightened by changing Occurs to Max Occurances. This eliminates having to parse the spec description's occurs column to infer if a field is required.