fun optCurrency(jsonObject: JSONObject, @Size(min = 1
) fieldName: String): String?
Calls through to JSONObject.optString while safely converting the raw string "null" and the empty string to
null, along with any value that isn't a three-character string.
a three-letter currency code if one is found, or
the object from which to retrieve the currency code
the name of the field in which the currency code is stored