Annotate ListStringTypeConverter for nullability

[MartyIX]

Description of Change

Recently, a PR was merged (#27984) where nullability annotations for ShadowTypeConverter were added. However, I believe that there are minor errors.

Based on dotnet/runtime#63874 which corrected nullable types for .NET runtime (e.g. DateTimeConverter), I believe that the correct method synopses are:

+public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
+public override bool CanConvertTo(ITypeDescriptorContext? context, Type? destinationType)
+public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
+public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)

This PR attempts to fix the annotations for a single converter to validate my beliefs.

---

If you wonder, what "~" character (called oblivious marker) denotes in PublicAPI.Shipped.txt and PublicAPI.Unshipped.txt files mean, then look here: https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md#nullable-reference-type-support

Any public API that haven't been annotated (i.e. uses an oblivious reference type) will be tracked with a ~ marker.
The marker lets you track how many public APIs still lack annotations. For instance, ~C.ObliviousMethod() -> string.

[dotnet-policy-service]

Hey there @MartyIX! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[bhavanesh2001]

This should be targeting main?

[MartyIX]

This should be targeting main?

I can retarget if it is not good for main.

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[jfversluis]

Not sure to what extend this is actually breaking, but just to be sure lets make this target .NET 10 since it does touch public APIs

[MartyIX]

I believe it's not a breaking change but I'm not 100 per cent sure.

I created #28244 for NET 10.0, given that tests run and I'm interested if there is an issue or not.

[jfversluis]

Lets keep this for .NET 10!