add check for isBuiltin method (for PHP 8 compatibility)

[karyna-tsymbal-atwix]

Description

Adds an additional check to avoid fatal error when using ReflectionUnionType::isBuiltin() method in PHP8
(see details in the related issue)

Related Issues

1. Check and update codebase to avoid fatal error for ReflectionType::isBuiltin() magento2#34194: Check and update codebase to avoid fatal error for ReflectionType::isBuiltin()

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/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[jilu1]

@magento import pull request to https://github.com/magento-commerce/magento2-functional-testing-framework

[m2-github-services]

@jilu1 the Pull Request is successfully imported.

[jilu1]

@karyna-tsymbal-atwix
Thank you for your PR! It will get merged soon.