Ensure we define the class property before using it

[dkotter]

Description of the Change

As reported, in newer versions of PHP (seems like 8.2+) a PHP deprecation is shown because of our NLU class. We have a class property nlu_features, that we use but never define. This PR fixes that by ensuring that property is defined.

Closes #547

How to test the Change

1. In an environment running PHP 8.2 and with this PR checked out, ensure no deprecation notices are shown
2. Ensure all NLU functionality still works
3. Downgrade to PHP 7.4 and ensure things still work as well

Changelog Entry

Fixed - Ensure we define a class property before using it to avoid PHP deprecation notices

Credits

Props @ankitguptaindia, @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.