MJCF to SDFormat: Do not create non-static models that have no links.

[azeey]

🦟 Bug fix

Summary

When converting an MJCF file with a single geom in worldbody, we currently end up creating a non-static model that has no links. But SDFormat requires all models except static ones to have at least one link. The solution here is to check if we've converted any bodies before adding the model to the world. I think it's okay to only check the number of links because model's can't have lights or sensors and joints would only be created if either the parent or child link is in the model.

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov[bot]]

Codecov Report

Merging #77 (7018325) into main (b3b464e) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #77   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files          24       24           
  Lines        1096     1097    +1     
=======================================
+ Hits         1043     1044    +1     
  Misses         53       53           

Impacted Files	Coverage Δ
...sdformat_mjcf/mjcf_to_sdformat/converters/world.py	98.71% <100.00%> (+0.01%)	:arrow_up:

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