Summary of the changes (Less than 80 chars)

• Fix source generator for nullable value type argument with explicit default value by considering Nullable
• Detail 2

Closes #bugnumber (in this specific format)

Background:
After switching from p-16.0.0-p.8.9 to p.10.1 the following results in a compile error CS0103 since the source code generator emits False as the value of HotChocolate.Types.Descriptors.Configurations.ArgumentConfiguration.RuntimeDefaultValue (instead of false )

[QueryType]
public static partial class TestQuery
{
    public static int GetFoo(bool? bar = false) => 42
}

Tests have also been added for other special cased value types.
One test was adjusted, as it is more correct yet than it was before.