Fix tutorial for `blender_sdf_exporter`

[AndrejOrsula]

🦟 Bug fix

Summary

I encountered 2 issues when trying the tutorial.

1. The COLLADA (.dae) would get exported correctly under ./meshes/model.dae. However, SDF file refer to ./model.dae as a relative URI for the mesh, which prevented Gazebo from finding it (assuming that only the parent directory of the model is part of GZ_SIM_RESOURCE_PATH). Corrected the path.
2. The script always expected a model to have an assigned material, which would make it fail for objects without any material. Added a simple check to prevent throwing an exception when exporting models without a material.

Tested both with Blender 2.82 and 3.2

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

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