Allow empty urdf tag (backport #1017)

[mergify]

This is an automatic backport of pull request #1017 done by Mergify.
Cherry-pick of 7c07430 has failed:

On branch mergify/bp/humble/pr-1017
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 7c07430.
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Changes to be committed:
	modified:   hardware_interface/test/test_component_parser.cpp
	modified:   ros2_control_test_assets/include/ros2_control_test_assets/components_urdfs.hpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   hardware_interface/src/component_parser.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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[fmauch]

As with #1029 there seem to have been problems while creating the backport using mergify.

[destogl]

Closing in favor of #1036