Fix no material issue in Export robot to URDF (pull/63)

[hyounesy]

Proposed change(s)

Applying the changes from original pull/63 as the file has changed significantly making it difficult to resolve the conflicts in the original PR.

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

https://github.com/Unity-Technologies/URDF-Importer/pull/63/

Provide any relevant links here.

Types of change(s)

• [x] Bug fix
• [ ] New feature
• [ ] Code refactor
• [ ] Documentation update
• [ ] Other (please describe)

Testing and Verification

Please describe the tests that you ran to verify your changes. Please also provide instructions, ROS packages, and Unity project files as appropriate so we can reproduce the test environment.

Test Configuration:

• Unity Version: Unity 2020.3.13f1]
• Unity machine OS + version: macOS 10.15.7

Checklist

• [x] Ensured this PR is up-to-date with the dev branch
• [x] Created this PR to target the dev branch
• [x] Followed the style guidelines as described in the Contribution Guidelines
• [ ] Added tests that prove my fix is effective or that my feature works
• [ ] Increased the test coverage criteria by 3%
• [ ] Updated the Changelog and described changes in the Unreleased section
• [ ] Updated the documentation as appropriate

Other comments