mac: Fallback empty codeset to UTF-8

Workaround for a very strange behavior in macOS (reported about 10.15
at least): `nl_langinfo(CODESET)` returns an empty string until the
parent shell sets `LC_CTYPE` (even if it doesn't export it).

localeinit.c

@@ -55,6 +55,9 @@ locale_charmap(VALUE (*conv)(const char *))
 #elif defined HAVE_LANGINFO_H
     codeset = nl_langinfo(CODESET);
     ASSUME(codeset);
+# ifdef __APPLE__
+    if (!*codeset) codeset = "UTF-8";
+# endif
 #else
 # error locale_charmap() is not implemented
 #endif