locale.c: ALT_DIGITS should be empty not "0" when none

This and the next commit fix the incorrect behavior that perl has always
had for nl_langinfo(ALT_DIGITS).

If there are no alternate digits, an empty string is returned by
nl_langinfo when there are no such alternate digits.  This should also
be the case on platforms where we emulate nl_langinfo, but instead "0"
was being returned.  This commit makes things consistent.

The next commit adds tests

Author: khwilliamson

locale.c

@@ -6830,7 +6830,7 @@ S_emulate_langinfo(pTHX_ const int item,
 #    endif
 #    if defined(WIN32) || ! defined(USE_LOCALE_TIME) || ! defined(HAS_STRFTIME)
 
-          case ALT_DIGITS: retval = "0"; break;
+          case ALT_DIGITS: retval = ""; break;
 #    else
           case ALT_DIGITS:
             format = "%Ow"; /* Find the alternate digit for 0 */