Fix AssimpLoader collada texture coordinates

[iche033]

🦟 Bug fix

Fixes #633

Summary

Fixes UNIT_AssimpLoader_TEST with assimp 5.4.3. This should fix homebrew CI.

This PR tweaks the way we check for texture coordinates from assimp. Empty texture coordinate slots are now allowed as of https://github.com/assimp/assimp/pull/5636. The new logic should be compatible with assimp 5.4.3 and prior versions

Checklist

• [x] 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.

[iche033]

Homebrew CI is fixed. This PR is ready for review.