[3.12] gh-72795: Make positional arguments with nargs='*' or REMAINDER non-required...

[3.12] gh-72795: Make positional arguments with nargs='*' or REMAINDER non-required (GH-124306) (GH-124422)

This allows to use positional argument with nargs='*' and without default
in mutually exclusive group and improves error message about required
arguments.
(cherry picked from commit 3c83f9958c14cd62ad8951c53536f7788745b0ba)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>