Datagrid: Throws NullReferenceException if SelectedItems is null during initialization

[karan-kang]

Describe the bug

Datagrid does not handle Null value for SelectedItems property during initialization.

To Reproduce
Steps to reproduce the behavior:

1. Follow usage in https://blazority.com/datagrid/multi-selection
2. Set SelectedUsers to Null during property initialization
3. Exception will be thrown

Unhandled exception rendering component: Object reference not set to an instance of an object.
      System.NullReferenceException: Object reference not set to an instance of an object.
         at Blazority.Docs.Pages.Demos.Datagrid.DemoDatagridSelectionMulti.<BuildRenderTree>b__0_4(RenderTreeBuilder __builder3)
         at Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder.AddContent(Int32 sequence, RenderFragment fragment)
         at Blazority.CardBlock.BuildRenderTree(RenderTreeBuilder __builder)
         at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment)
         at Microsoft.AspNetCore.Components.RenderTree.Renderer.RenderInExistingBatch(RenderQueueEntry renderQueueEntry)
         at Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue()

Environment

• Blazority Version: 0.8.1
• .NET Core Version: 5.0
• Browser: Chrome