Fix Critical error on create new test default list of features in the selected behat features folder

Problem/Motivation

TypeError: array_key_first(): Argument #1 ($array) must be of type array, string given in array_key_first() (line 244 of modules/contrib/behat_ui/src/Form/BehatUiNew.php).

Steps to reproduce

Go to admin/config/development/behat-ui/new

Proposed resolution

Cast the default value as an array

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Merge request, Patch, or Commit
• ✅ Testing to ensure no regression
• ➖ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Documentation
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Release notes snippet
• ❌ Release behat_ui-4.0.3

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3300399 by fsabbagh, Vladimir Tanovic: Fixed Critical error on create new test default list of features in the selected behat features folder