[3.13] gh-138568: Make `help` mode in PyREPL not exit on empty line input (GH-143512) (GH-143520)

(cherry picked from commit b3e4a3462f6011cacdfc58c565c69e4d7de6b9cd)

Signed-off-by: yihong0618 <zouzou0208@gmail.com>
Co-authored-by: yihong0618 <zouzou0208@gmail.com>