[3.12] gh-124130: Notes on empty string corner case of category `\B` (GH-124133) (#124329)

gh-124130: Notes on empty string corner case of category `\B` (GH-124133)
(cherry picked from commit d3e79d75d164c338a64fd66edb26e69c501cee58)

Signed-off-by: y5c4l3 <y5c4l3@proton.me>
Co-authored-by: Y5 <124019959+y5c4l3@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>