Initialize all outgoing vehicle_command_ack_s and vehicle_command_s

This will initialize those structs with zero in all fields not set
and all fields set will only be change once to the final value not
wasting CPU time zeroing it.

This will guarantee that no non-unitialized structs will have
a trash value on from_external causing it to be sent to the
MAVLink channel without need it.

src/modules/uavcan/uavcan_servers.cpp

@@ -534,9 +534,11 @@ pthread_addr_t UavcanServers::run(pthread_addr_t)
 			}
 
 			// Acknowledge the received command
-			struct vehicle_command_ack_s ack = {};
-			ack.command = cmd.command;
-			ack.result = cmd_ack_result;
+			struct vehicle_command_ack_s ack = {
+				.timestamp = 0,
+				.command = cmd.command,
+				.result = cmd_ack_result
+			};
 
 			if (_command_ack_pub == nullptr) {
 				_command_ack_pub = orb_advertise_queue(ORB_ID(vehicle_command_ack), &ack, vehicle_command_ack_s::ORB_QUEUE_LENGTH);