Add proper handling of NULL data return

[rmbielby]

Is your feature request related to a problem? Please describe.

If a query is legitimate in terms of what the API will accept, but there's just no data rows corresponding to the query terms, then the API returns a 200 response, but with no results. This gets pulled out as a NULL in post_dataset / get_dataset and the code errors with something like select not applicable to object NULL.

Examples of this are:

• If the user requests a geographic level that's legitimate, but not present in the data set e.g. "SCH" in a data set only containing national data.

Describe the solution you'd like

A meaningful error message saying no data was found. Could even do a check on the obvious causes like "SCH is not found as a geographic level in this data file".