[ros2interface] shows empty comments even with --no-comments

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04
• Installation type:
  • binaries
• Version or commit hash:
  • 0.13.4 (galactic)
• DDS implementation:
  • N/A
• Client library (if applicable):
  • N/A

Steps to reproduce issue

ros2 interface show tf2_msgs/msg/TFMessage --no-comments

Actually, the --no-comments param is not necessary, because it's the default behavior. Just included here for clarification.

Expected behavior

ros2interface should strip all comments:

$ ros2 interface show tf2_msgs/msg/TFMessage --no-comments
geometry_msgs/TransformStamped[] transforms
	std_msgs/Header header
		builtin_interfaces/Time stamp
			int32 sec
			uint32 nanosec
		string frame_id
	string child_frame_id
	Transform transform
		Vector3 translation
			float64 x
			float64 y
			float64 z
		Quaternion rotation
			float64 x 0
			float64 y 0
			float64 z 0
			float64 w 1

Actual behavior

ros2interface does not strip empty comment lines (see the two lines starting with #). This is because of the empty comment lines in tf2_msgs/msg/TFMessage:

$ ros2 interface show tf2_msgs/msg/TFMessage --no-comments
geometry_msgs/TransformStamped[] transforms
	#
	#
	std_msgs/Header header
		builtin_interfaces/Time stamp
			int32 sec
			uint32 nanosec
		string frame_id
	string child_frame_id
	Transform transform
		Vector3 translation
			float64 x
			float64 y
			float64 z
		Quaternion rotation
			float64 x 0
			float64 y 0
			float64 z 0
			float64 w 1

Additional information

Start by running ros2 interface show tf2_msgs/msg/TFMessage --no-comments on the reported Galactic setup, then trace the ros2interface entry point that filters comments. Done means the output has no empty # lines while preserving the shown fields; add coverage for this TFMessage case if the existing tests provide a suitable location.