Unable to map nullable primitive types (string? to bool?) using custom MapWith delegate

A pull request for this has already been merged.

• #993 by @DocSvartz — merged
• #999 by @DocSvartz — merged

What am I missing?

```csharp
using Mapster;
using MapsterMapper;

TypeAdapterConfig config = new TypeAdapterConfig();
config.NewConfig().MapWith(src => Helper.ParseToNullableBool(src));
config.Compile();

Mapper mapper = new Mapper(config);

Source source = new Source { Prop1 = "yes" };
Destination destination = mapper.Map(source);

Console.ReadLine();

static class Helper
{
public static bool? ParseToNullableBool(string? value)
{
if (string.IsNullOrWhiteSpace(value))
return null;

return value.Trim().ToLower() switch
{
"true" or "t" or "yes" or "1" => true,
"false" or "f" or "no" or "0" => false,
_ => null
};
}
}

class Source
{
public string? Prop1 { get; set; }
}

class Destination
{
public bool? Prop1 { get; set; }
}
```

converting non-nullable string to non-nullable bool works fine

Reproduce the issue using the nullable string-to-bool MapWith configuration and the Source and Destination classes shown in the report. Start by tracing how MapWith delegates are compiled for nullable primitive properties; done means the custom delegate maps Prop1 correctly, including null and invalid values, while the existing non-nullable case remains working.

Written by the indexing model from the issue text.