[3.14] pythongh-133740: Fix regression in locale.nl_langinfo(ALT_DIGITS) (pythonGH-136237) (pythonGH-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 5c984ae)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: miss-islington

Modules/_localemodule.c

@@ -692,7 +692,7 @@ _locale_nl_langinfo_impl(PyObject *module, int item)
             result = result != NULL ? result : "";
             char *oldloc = NULL;
             if (langinfo_constants[i].category != LC_CTYPE
-                && (
+                && *result && (
 #ifdef __GLIBC__
                     // gh-133740: Always change the locale for ALT_DIGITS and ERA
 #  ifdef ALT_DIGITS