Fix: handle exception in get_link_names for single-link group

[annapurna-gupta]

This pull request addresses an issue with the get_link_names method in the compas_fab library. Previously, this method threw an exception when called for groups containing only one link, indicating that no chain was found. The fix ensures that this method can now handle such cases without raising errors, allowing users to retrieve link names for single-link groups successfully.

This PR fixes #160

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

##checklist

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• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas_fab.robots.CollisionMesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)