Fix defaults for optionals, remove placeholder exhibits, do not add section if empty

[krassowski]

Reference Issues or PRs

• not-failing on optional value being None is needed for Implement support for jupyterlab-gallery config nebari#2501
• not adding a section to launcher if no exhibits are configured is a short-term solution for Should the gallery section be hidden in the launcher if no exhibits are configured? #18

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

[github-actions]

👈 Launch a Binder on branch nebari-dev/jupyterlab-gallery/fix-defaults