gdatetime: Fix string type used to initialise array

This fixes commit 057f0fc. I didn’t
notice that `tmp` is an array of strings, not an array of chars, and
somehow my compiler didn’t warn. Seems only the macOS CI job is spotting
the problem here.

Signed-off-by: Philip Withnall <[email protected]>

glib/gdatetime.c

@@ -3104,7 +3104,7 @@ format_number (GString     *str,
     "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
   };
   const gchar * const *digits = ascii_digits;
-  const gchar *tmp[10] = { '\0', };
+  const gchar *tmp[10] = { NULL, };
   gint i = 0;
 #ifdef HAVE_LANGINFO_OUTDIGIT
   static GMutex alt_digits_mutex;