This patch improves handling of NULLs in textual fields in ColumnStore.
Previously empty strings were considered NULLs and it could be a problem
if data scheme allows for empty strings. It was also one of major
reasons of behavior difference between ColumnStore and other engines in
MariaDB family.
Also, this patch fixes some other bugs and incorrect behavior, for
example, incorrect comparison for "column <= ''" which evaluates to
constant True for all purposes before this patch.
NULL is now pushed through the MariaDB storage engine plugin down to the
insert processing. A '0000-00-00' date is now a separate value to NULL.
This is more in-line with MariaDB's handling.
