[3.14] gh-133740: Fix regression in locale.nl_langinfo(ALT_DIGITS) (GH-136237) (GH-136240)

There is no need to temporary switch locale for items ALT_DIGITS and ERA
if the nl_langinfo() result is empty (most locales).
(cherry picked from commit 5c984ae35e30f0533e6cc727d23cc158decf001c)

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