Updated the TransformerInterface

[danielmorell]

Description of the change

In this PR I have added comments to the TransformerInterface.

I also updated the return type on the TransformerInterface::transform() method from a union of null and Payload to just be Payload. The reason for this is, a null return type has not been accounted for in the logic flow where the payload is used. It is expected to be a Payload instance not null.

I cannot think of a reason to transform the payload to null other than to have the error ignored. However, check ignore is the very next step. So, that does not seem like plausible need either.

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]

Rationale to remove null makes sense. Thanks for the explanation.