[Bug-Fix] Fixing inability to read relative file paths

Proposed change(s)

Fix a regression from a previous pull request where the function UrdfAssetPathHandler.GetRelativeAssetPathFromUrdfPath was returning null in case the file path is of format "folder1/folder2/example.stl".

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

https://github.com/Unity-Technologies/URDF-Importer/pull/147/files#r718991313 https://jira.unity3d.com/browse/AIRO-1349

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: [e.g. Unity 2020.2.0f1]
Unity machine OS + version: [e.g. Windows 10]
ROS machine OS + version: [e.g. Ubuntu 18.04, ROS Noetic]
ROS–Unity communication: [e.g. Docker]

Checklist

[x] Ensured this PR is up-to-date with the dev branch
[x] Created this PR to target the dev branch
[ ] 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

Unity-Technologies / URDF-Importer

[Bug-Fix] Fixing inability to read relative file paths #150