Checking nullability of test properties before setting to null in integration test framework cleanup

[stollr]

Description (*)

Previously an error occured if a non-nullable typed property was used in a test case. The error message was something like

Cannot assign null to property MyVendor\MyModule\Test\Integration\MyTest::$objectManager of type Magento\Framework\ObjectManagerInterface

Manual testing scenarios (*)

1. Add a non-nullable typed property to an integration test (like the one in the DummyTestCase in this PR)
2. Run the test
3. In the test result there will occur an error message looking like the above one.

Contribution checklist (*)

• [x] Pull request has a meaningful description of its purpose
• [x] All commits are accompanied by meaningful commit messages
• [x] All new or changed code is covered with unit/integration tests (if applicable)
• [x] README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• [ ] All automated tests passed successfully (all builds are green)

Resolved issues:

1. [x] resolves magento/magento2#39245: Checking nullability of test properties before setting to null in integration test framework cleanup

[m2-assistant[bot]]

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[engcom-Hotel]

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