dynsec: setClientId should cope with missing/empty client id.

plugins/dynamic-security/clients.c

@@ -575,11 +575,15 @@ int dynsec_clients__process_set_id(cJSON *j_responses, struct mosquitto *context
 		return MOSQ_ERR_SUCCESS;
 	}
 
+	if(clientid != NULL && strlen(clientid) > 0){
 		clientid_heap = mosquitto_strdup(clientid);
 		if(clientid_heap == NULL){
 			dynsec__command_reply(j_responses, context, "setClientId", "Internal error", correlation_data);
 			return MOSQ_ERR_NOMEM;
 		}
+	}else{
+		clientid_heap = NULL;
+	}
 
 	mosquitto_free(client->clientid);
 	client->clientid = clientid_heap;