BindableCache: nested nullable objects fail converting to key-value-pair

[g105b]

When there's an object that has a nullable public property, like so:

class Customer {
  public function __construct(
    public string $name,
    public ?Address $address,
  ) {}
}

If the Customer's address is null, BindableCache::convertToKvp() will try to use getAddress() instead of being happy with a null value.

[g105b]

Before closing, it would be good to link the unit test here, as this latest merge is becoming a bit confusing :S