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

**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.