Don't record sim-time messages before first /clock (backport #1354)

[mergify]

This is an automatic backport of pull request #1354 done by Mergify.
Cherry-pick of da7528f has failed:

On branch mergify/bp/humble/pr-1354
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit da7528f.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   README.md

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ros2bag/ros2bag/verb/record.py
	both modified:   rosbag2_transport/src/rosbag2_transport/recorder.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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