This patch is the JSON_ARRAYAGG clone of the changes done in MCOL-5429
where we enabled usage of StringStore for long strings in
GROUP_CONCAT() processing to reduce memory footprint of PrimProc and
thus avoiding a potential OS triggered OOM crash.
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.
