Fix crash for missing urdf in resource manager (backport #1301)

[mergify]

This is an automatic backport of pull request #1301 done by Mergify.
Cherry-pick of 510ba73 has failed:

On branch mergify/bp/humble/pr-1301
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Changes to be committed:
	modified:   hardware_interface/include/hardware_interface/resource_manager.hpp

Unmerged paths:
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	both modified:   hardware_interface/src/resource_manager.cpp

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[saikishor]

I fixed the conflicts of the humble PR in a new PR : #1316