RowBasedKeySerde should use empty dictionary in constructor

[gaodayue]

Fixes #6255

[gianm]

LGTM. Could you please add a comment about why the map is starting out empty (avoiding allocating too much when it's not needed)? Otherwise, a future contributor might not realize it.

[jihoonson]

Hi @gaodayue, I left a comment on #6255.

[jihoonson]

I've checked more around RowBasedKeySerde and noticed that it's initialized to 10000 no matter whether spilling is enabled or not in ConcurrentGrouper. Also, it's always initialized no matter what the column type is even though it's being used for only string columns. Probably we can optimize this further, but it looks a different issue. This patch looks good to me. Thanks @gaodayue!

[clintropolis]

We should probably backport this to 0.12.3

[gaodayue]

Could you please add a comment about why the map is starting out empty (avoiding allocating too much when it's not needed)? Otherwise, a future contributor might not realize it.

Thank you @gianm . It's a good advice, but the PR is merged. Maybe we can add it in future optimization.