[3.12] gh-127563: use `dk_log2_index_bytes=3` in empty dicts (GH-127568) (GH-127813)

This fixes a UBSan failure (unaligned zero-size memcpy) in `dictobject.c`.
(cherry picked from commit 9af96f440618304e7cc609c246e1f8c8b2d7a119)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>