Added new feature to force empty values to nil

[lsadehaan]

The new parameter column_names_empty_to_null is used to specify columns for which an empty parameter should be forced to nil (so it becomes a null when inserting to the DB).
This was essential for me where I had a CSV formatted log file I needed to push to various DB columns, but some of the CSV parameters could be "", for things like dates or int's resulting in errors during bulk insert.
With this version you just add those columns to column_names_empty_to_null and they are inserted as NULL to the table.
Perhaps a future improvement could be the possibility to have a mapping of input values to output values - for instance other inputs like - or * or "" could all be considered null. This version I kept simple though.