Do not pass null string to %s printf formatted string

Here the string is already decided to be NULL and passing
a null pointer to %s will not work

Fixes
error: '%s' directive argument is null

Signed-off-by: Khem Raj <raj.khem@gmail.com>

plugins/mtk/mm-broadband-modem-mtk.c

@@ -190,7 +190,7 @@ get_supported_modes_ready (MMBaseModem *self,
 
     response = mm_base_modem_at_command_finish (MM_BASE_MODEM (self), res, &error);
     if (!response) {
-        mm_dbg ("Fail to get response %s", response);
+        mm_dbg ("Fail to get response");
         g_task_return_error (task, error);
         g_object_unref (task);
         return;