Thank you for your responses.
Yes, I have tried using both length and empty string checks, and that works for string properties. However, null checks are supposed to work for any type such as numbers.
Trying that approach to filter an Integer property, for instance, named NUM_SPOTS with $filter=NUM_SPOTS eq ''
results in this error:
Operator 'eq' incompatible with operand types 'Edm.Int32' and 'Edm.String'.
I take that as the null literal construct the OData spec mentions isn't implemented, correct?