[BUGFIX]--fix replicaCAD quickstart to not default to showing the NONE scene

[jturner65]

Motivation and Context

This addresses ReplicaCAD_quickstart.py only showing the NONE scene when run (the code would default to using the first scene in the list of available scenes, which would be the NONE scene since they are alphabetically sorted). Now the NONE scene is returned only if, for some reason, the list of scenes is empty. This is only an issue when running the code in python, since the drop-down scene select is not available there.

How Has This Been Tested

Locally tested both python and colab.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[aclegg3]

LGTM

[dhruvbatra]

Yup, I get a video now. It looks like the agent was spawned below the stairs or in a corner because all I saw was gray, but a scene was loaded.

Let's land this and I can test again after rebasing.