SDFormat to MJCF: Fix bug with plane sizes

[azeey]

🦟 Bug fix

Summary

This was brought up by @scpeters in https://github.com/gazebosim/gz-mujoco/pull/2/files/d0eaa013fd4a68b5fe5c172f3042278808f0a9f1#r866318057 and I responded saying setting it 0 causes Mujoco to fail, but somehow I merged the PR without fixing it.

This sets the third component of a plane's size to 1. I don't have a good reason for setting it that, but it fixes the failure for now. This could be configurable via command line flag.

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)
• [x] 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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

Merging #54 (0bede6f) into main (20d9653) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files          16       16           
  Lines         628      628           
=======================================
  Hits          614      614           
  Misses         14       14           

Impacted Files	Coverage Δ
...at_to_mjcf/sdformat_to_mjcf/converters/geometry.py	92.06% <100.00%> (ø)

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