`NA`, `np.nan`, and `pd.NA` are not valid JSON standard, return `null` instead

[jmillanacosta]

Describe the bug
The annotators return NA values for the output_dict values when no annotation was available. They should return null to avoid downstream issues (e.g. if using BDF as an API from javascript).

Expected behavior
Replace all NA-like returns for null so we can do json.dumps() with BDF output.