Added typing / comments to the ScrubberInterface and Scrubber class

[danielmorell]

Description of the change

This PR adds better typing and comments to the ScrubberInterface to make implementation less error prone. In the process it seemed best to add these same updates to the Scrubber class as well.

The update of the Scrubber class required some additional updates around the serialization of the Payload class. Since the Scubber::scrub() method is only ever given the output of Payload::serialize(), I updated the scrub() method to only accept an array as the $data argument. This is stricter than it was previously, however, the only usage that deviates from array is in the tests. Additionally, if scrubbing of other types is needed, the Scrubber::internalScrub() method is unlike its name would lead you to believe public.

Other changes

1. The MalformedPayloadDataTransformer serialized payload was changed from false to an empty array.
2. The previous change caused the response HTTP status code to change from 400 to 422.
3. I cleaned up the logic and local variable usage of the anonymous function used for array_walk() in Scrubber::scrubArray().

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

None

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[waltjones]

👍