Fix sending null behavor for empty strings, remove testing hardcode, some refactoring of add_batches

* Sending nulls worked only for nil values previously, now works for empty strings.
* Sending nulls worked only for fields with a string data type, now works for all fields.
* Remove hardcode from send nulls behavior that was inadvertently left in.
* Break down add_batches as start to refactoring.
* Add ability to selectively send null values.