Allowed expressions should allow simple defaults

[dsyme]

A generated workflow used a nice pattern to embed defaults reliably:

- **Repository**: ${{ github.repository }}
- **Run ID**: ${{ github.run_id }}
- **Triggered by**: @${{ github.actor }}
- **Devices to test**: ${{ inputs.devices || 'mobile,tablet,desktop' }}
- **Docs directory**: ${{ inputs.docs_dir || 'docs' }}
- **Build command**: ${{ inputs.build_command || 'npm run build' }}
- **Serve command**: ${{ inputs.serve_command || 'npm run preview' }}
- **Server port**: ${{ inputs.server_port || '4321' }}
- **Working directory**: ${{ github.workspace }}

This seems valid to allow - a || b for two valid expressions including simple constants