Patch: FORCE_NULL option for copy COPY in CSV mode

Hi,

This patch implements the following TODO item:

  Allow COPY in CSV mode to control whether a quoted zero-length
  string is treated as NULL

      Currently this is always treated as a zero-length string,
      which generates an error when loading into an integer column

        Re: [PATCHES] allow CSV quote in NULL
        http://archives.postgresql.org/pgsql-hackers/2007-07/msg00905.php


  http://wiki.postgresql.org/wiki/Todo#COPY


I had a very definite use-case for this functionality recently while importing
CSV files generated by Oracle, and was somewhat frustrated by the existence
of a FORCE_NOT_NULL option for specific columns, but not one for
FORCE_NULL.

I'll add this to the November commitfest.


Regards

Ian Barwick