Display `null` values as `null`, not None

[duncandewhurst]

In the first 3 examples column, null values are displayed as None (from Python), which is confusing because that's not what null values look like in JSON data. It would be clearer if they were displayed as a code-formatted null.

Data

{
  "description": null
}

Current output

Suggested output

[jpmckinney]

Essentially same as open-contracting/cove-ocds#56 (i.e. use the JSON representation, not the Python string representation).