Create nimble-mapping-empty component

[mollykreis]

Pull Request

🤨 Rationale

As part of #1992, we decided to create a nimble-mapping-empty component that would allow a cell to render as blank for a given mapping while still having text provided for use in group rows.

This PR creates that component within nimble-components. Support for Blazor and Angular will be added in a subsequent PR.

👩‍💻 Implementation

• Create nimble-mapping-empty component
• Add support for the component in the mapping column
• Update tests

🧪 Testing

• Extended the unit tests for the mapping column to include tests for empty mappings
• Add empty mapping to the matrix test for the mapping column

✅ Checklist

• [ ] I have updated the project documentation to reflect my changes or determined no changes are needed.

[mollykreis]

@m-akinc, will you buddy this PR for me?