Fix bug where included URDFs are not parsed when there are no custom parsers

[azeey]

🦟 Bug fix

Summary

#475 introduced a regression where included URDFs were being silently ignored when there are no custom parsers available. This returns the behavior back to URDFs being parsed natively by SDFormat if there are no custom parsers.

Note, however, that if there are custom parsers, but none of them are able to parse the URDF, the URDF file will be ignored. This might not be desirable, but changing the logic so that URDFs get parsed by SDFormat after trying all available custom parsers will require more significant changes, so I did not attempt to do that in this PR.

Checklist

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• Added tests
• Updated documentation (as needed)
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