[DataGrid] Respecting control state of Loading parameter

[adamint]

Pull Request

📖 Description

🎫 Issues

Fixes #2998

👩‍💻 Reviewer Notes

This PR changes FluentDataGrid.Loading to a nullable boolean. If the value is not-null, the grid is considered controlled and requires the user to change loading state themselves. If the value is null, the grid will manage loading state itself - if Items are provided, Loading is set to false, otherwise Loading is true until the ItemsProvider has returned data.

This also fixes the issue where initially EmptyContent is rendered if there is an ItemsProvider when the grid should be rendering LoadingContent.

As a smoke test, you can look at the "Remote Data" section on the DataGrid demo example page, which now shows a loading page before content is loaded.

📑 Test Plan

I've added a basic DataGrid test as well as several tests surrounding Loading, with either ItemsProvider or Items.

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps