use options.EMPTY_FIELD_VALUE when '', null, undefined about json-2-csv

[mrodrig]

Need to add to convertData function:
if (_.isUndefined(fieldData) || fieldData == '' || fieldData == null) { fieldData = options.EMPTY_FIELD_VALUE; }

Adding to v3.0.0

[mrodrig]

This is no longer a problem in 3.0.0 which will hopefully be out in a month or so. Closing this for now since the code is completely revamped and won't have this problem.