gh-96848: Fix -X int_max_str_digits option parsing (GH-96988)

Fix command line parsing: reject "-X int_max_str_digits" option with
no value (invalid) when the PYTHONINTMAXSTRDIGITS environment
variable is set to a valid limit.
(cherry picked from commit 41351662)

Co-authored-by: Victor Stinner <vstinner@python.org>