NA Representation not Applying to Score/Count CSVs

[bencap]

When querying the Score and Count CSV endpoints, the NA representation is not being properly applied. This results in null values being displayed with None (the stringified Python type) rather than our standard null representation of NA.

To reproduce:

• Query the CSV endpoint for a score set which has null scores
• Observe these scores are represented with the string None

We should fix the representation and add additional tests to ensure the NA rep is applying correctly for all CSV exports.