Php8.4

[phpfui]

Fixes

This PR makes the minimal changes needed to avoid depreciation messages for PHP 8.4. It does not contain any logic changes:

• Removes E_STRING in tests/bootstrap.php
• Add ? to parameter types that default to null
• Make explict $value property in \Twilio\TwiML\GenericNode
• Change assertObjectNotHasAttribute to assertObjectNotHasProperty for PHPUnit

All changes should be compatible with PHP 7.1, which is the lowest supported PHP version.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.