[backport] fixes for instant purchase module from #15257

[mhauri]

Description

Original PR: #15257

The Method get($type) in /Magento/InstantPurchase/Model/ShippingMethodChoose/DeferredShippingMethodChooserPool.php throws an Exception without showing the shipping method $type as it was hardcoded as 'chooser'.

Allowing null for properties in InstantPurchaseOption.php as they are null in the constructor by default.

Fixing PHPDocs as they refer to wrong Classes or have missing type declarations.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @miguelbalparda, thank you for the review.
ENGCOM-1579 has been created to process this Pull Request

[magento-engcom-team]

@mhauri thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[mhauri]

Thank you @magento-engcom-team for the invitation 🎉

[mhauri]

@miguelbalparda may you have time to also review the original PR: #15257

[magento-engcom-team]

Hi @mhauri. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.5 release.

Please, consider porting this solution across release lines.
You may use Porting tool to port commits automatically.