Commits
Chenxi Li authored and David Li committed 39367db2dab
ARROW-15126: [C++] Support Null type as group keys Closes #12032 from Crystrix/arrow-15126 Lead-authored-by: Chenxi Li <chenxi.li@live.com> Co-authored-by: crystrix <chenxi.li@live.com> Signed-off-by: David Li <li.davidm96@gmail.com>