Do not try to export builtin textures

[azeey]

🦟 Bug fix

Summary

When exporting textures, if the texture is a builtin (eg. checker), the converter raises

 AttributeError: 'NoneType' object has no attribute 'prefix'

This PR fixes it by checking if the file attribute is present and set to a non-None value.

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] 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.

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[codecov[bot]]

Codecov Report

Merging #96 (e360bd0) into main (03ac0d4) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   95.11%   95.11%           
=======================================
  Files          24       24           
  Lines        1167     1168    +1     
=======================================
+ Hits         1110     1111    +1     
  Misses         57       57           

Impacted Files	Coverage Δ
...sdformat_mjcf/mjcf_to_sdformat/mjcf_to_sdformat.py	100.00% <100.00%> (ø)

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