serial: fix crash when sending some commands to a closed port (lp:963102)

If the command we're sending doesn't have a callback, don't try
to call NULL.  Triggered if the port got closed (because the modem
crashed, or refcounting errors or whatever) with some code like this:

    mm_at_serial_port_queue_command (MM_AT_SERIAL_PORT (port), "+CREG=0", 3, NULL, NULL);

src/mm-serial-port.c

@@ -1102,7 +1102,8 @@ internal_queue_command (MMSerialPort *self,
         GError *error = g_error_new_literal (MM_SERIAL_ERROR,
                                              MM_SERIAL_ERROR_SEND_FAILED,
                                              "Sending command failed: device is not enabled");
-        callback (self, NULL, error, user_data);
+        if (callback)
+            callback (self, NULL, error, user_data);
         g_error_free (error);
         return;
     }