Adjust NULL string tracing (ARMmbed#1726)

IAR toolchain fails to print NULL string traces. Adjust trace
to print (none) when string pointer is NULL.

Author: Arto Kinnunen

source/6LoWPAN/Thread/thread_leader_service.c

@@ -1090,7 +1090,7 @@ static int thread_leader_service_petition_cb(int8_t service_id, uint8_t source_a
         ptr = thread_meshcop_tlv_data_write_uint8(ptr, MESHCOP_TLV_STATE, 0xff);
     }
 
-    tr_debug("Petition req recv id %s, RESP session id: %d ret %d", commissioner_id_ptr, session_id, ret);
+    tr_debug("Petition req recv id %s, RESP session id: %d ret %d", commissioner_id_ptr ? commissioner_id_ptr : "(none)", session_id, ret);
 
 send_error_response:
     coap_service_response_send(this->coap_service_id, COAP_REQUEST_OPTIONS_NONE, request_ptr, response_code, COAP_CT_OCTET_STREAM, payload, ptr - payload);