Nullable cascading type argument should warn when used

[jjonescz]

Originally discussed in #8651 (comment).

using Microsoft.AspNetCore.Components;

namespace Test;

[CascadingTypeParameter(nameof(TRow))]
public class Parent<TRow>: ComponentBase
{
    [Parameter]
    public RenderFragment<TRow>? ChildContent { get; set; }
}

public class Child<TRow> : ComponentBase
{
    [Parameter]
    public RenderFragment<TRow>? ChildContent { get; set; }
}

<Parent TRow="string?">
    <Child Context="childContext">@childContext.Length</Child>
</Parent>

Apart from there being warnings (tracked in #7398):

warning CS8669: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

and in design-time builds even errors (see #8692),
there is a missing warning CS8602: Dereference of a possibly null reference..

It was originally suggested by @jsakamoto in #7428 (comment) to use default(Action<T>) instead of default(T) in code generation to support nullable reference types for T - seems like it could work (need to think about type constraints - are errors reported correctly if the cascading type parameter has type constraints which are not met?).