Work out reconciliation process for retrieve providers

"Inject prefetch resources some other way, like parameters in CQL", but that breaks a whole bunch of ecosystem stuff.
null = unknown empty
empty = known empty
otherwise = known non-empty.

That obviously has potentially far-reaching implications for engine behavior. But I think a first pass is not too bad.
Null dataProvider result =
new CqlException("DataProvider was unable to determine if retrieve XXX was null or empty. Please ensure the evaluation environment has all data necessary to make that determination")

Then in the context of prefetch, we can fall back. In the context of using a REST provider directly, something went horribly wrong.

Start by tracing how retrieve providers return data and how CQL prefetch resources are supplied. Clarify the distinction between null, known empty, and known non-empty results, then determine the intended fallback behavior for prefetch and direct REST-provider use. Done means the reconciliation process and its engine implications are agreed and documented in a form that can guide implementation.