feat(pointcloud_preprocessor): enable to count empty pointclouds topic in concatenate pointclouds nodelet

[YoshiRi]

Description

Problem

• concatenate process exit with completing topics subscription or timeout
• topic with empty pointclouds are returned and not counted as subscribed, so that the concatenate node will wait until timeout. This will cause a slight delay.
  • Our XX1 vehicle faces lower topic rate with this problem

PR solutions

• enable concatenate node to count empty pointclouds as well
• Need some escape for the latter concatenation process

Related links

TIER IV INTERNAL SLACK DISCUSSION

Tests performed

Tested with jpntaxi settings and rosbag.

Notes for reviewers

Interface changes

Effects on system behavior

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[iwatake2222]

@YoshiRi
I confirmed the delay was improved when one of input topics of concatenate node is empty.
The following logs show the frequency of output topic ( /sensing/lidar/concatenated/pointcloud )

• Original code

$ ros2 topic hz /sensing/lidar/concatenated/pointcloud -w 10
average rate: 7.855
	min: 0.096s max: 0.179s std dev: 0.02487s window: 9
average rate: 8.227
	min: 0.096s max: 0.163s std dev: 0.02371s window: 10
average rate: 8.633
	min: 0.088s max: 0.165s std dev: 0.02208s window: 10

• With this PR

$ ros2 topic hz /sensing/lidar/concatenated/pointcloud -w 10
average rate: 9.987
	min: 0.083s max: 0.114s std dev: 0.00757s window: 10
average rate: 10.039
	min: 0.049s max: 0.151s std dev: 0.02364s window: 10
average rate: 9.926
	min: 0.091s max: 0.112s std dev: 0.00576s window: 10

[miursh]

LGTM
confirmed with topic hz
before:

average rate: 8.285
	min: 0.100s max: 0.164s std dev: 0.02018s window: 10
average rate: 8.328
	min: 0.099s max: 0.157s std dev: 0.01780s window: 10
average rate: 7.838
	min: 0.100s max: 0.179s std dev: 0.02874s window: 10

after

average rate: 10.014
	min: 0.082s max: 0.114s std dev: 0.00906s window: 10
average rate: 10.078
	min: 0.090s max: 0.110s std dev: 0.00518s window: 10
average rate: 9.975
	min: 0.094s max: 0.107s std dev: 0.00405s window: 10