Default values in target are not handled

[nikophil]

This PR made me realize there is another problem, a little bit more complex to mitigate:

readonly class SomeClass
{
    public function __construct(public int|null $foo = 1, public int $bar = 0)
    {
    }
}

$autoMapper->map([], SomeClass::class);

two problems here:

• foo defaults to null whereas it should default to 1
• the above code creates an error because key bar is not present, although the property has a default value

I'm not sure how we could fix this... maybe by adding the default value in the WriteMutator?