[Blazor] Preserve null values for <option value="@null"> in single select binding

[Yuvan111]

[Blazor] Preserve null values for in single select binding

Description

<option value="@null"> now binds to a null value on a single <select>, instead of being treated as an empty string. The DOM cannot represent a null <option> value, so we render the value as an empty string and add a small marker attribute that the change handler recognizes. The result is reported to .NET as null, so a nullable bound property receives null directly no manual conversion from "" is needed.

Html Output:

Scope and limitations

• Single <select> only. Multi-select is not supported in this change, because the array of selected values is delivered as a plain string[] and cannot carry null entries end-to-end.
• Don't mix <option value=""> and <option value="@null"> in the same <select>. They render identically in the DOM (both as value=""); only the marker-bearing option is reported as null on the change event. The empty-string option will arrive as "". This is by design the two options are indistinguishable to the user and to the DOM, but only the marker option carries the null meaning back to .NET.
• Nullable target only. The feature is meaningful when the bound property is string?. Binding to a non-nullable string will receive null and surface a runtime error, as expected.
• Existing behavior preserved. <select> without <option value="@null"> continues to report the element's value exactly as before, including the "" returned when nothing is selected.

Fixes #23934

[dotnet-policy-service]

Thanks for your PR, @Yuvan111. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.