Fix generation of next gen images with first settings save of avif option

[wordpressfan]

Description

Run next gen generation for the first settings save and add few comments in code to explain what is going on there.

Fixes #825

Documentation

User documentation

This will trigger generating next generation images when enabling the avif option and rewrite rules.

Technical documentation

Explain how this code works. Diagram & drawings are welcomed.

Type of change

Delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue).

New dependencies

List any new dependencies that are required for this change.

Risks

List possible performance & security issues or risks, and explain how they have been mitigated.

Checklists

Feature validation

• I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Documentation

• I prepared the user documentation for the feature/enhancement and shared it in the PR or the GitHub issue.
• The user documentation covers new/changed entry points (endpoints, WP hooks, configuration files, ...).
• I prepared the technical documentation if needed, and shared it in the PR or the GitHub issue.

Code style

• I wrote self-explanatory code about what it does.
• I wrote comments to explain why it does it.
• I named variables and functions explicitely.
• I protected entry points against unexpected inputs.
• I did not introduce unecessary complexity.
• I listed the introduced external dependencies explicitely on the PR.
• I validated the repo-specific guidelines from CONTRIBUTING.md.

Observability

• I handled errors when needed.
•  I wrote user-facing messages that are understandable and provide actionable feedbacks.
• I prepared ways to observe the implemented system (logs, data, etc.).

Risks

•  I explicitely mentioned performance risks in the PR.
• I explicitely mentioned security risks in the PR.

[MathieuLamiot]

Don't block the PR for this, but it could be nice to update the doc string or comment the method here to clearly state:

• the expected behavior (only run when moving the avif conversion for off to on)
• the parameter's expected value (not set / 1)
  That could avoid some headache next time we work on this :D

[Mai-Saad]

Now AVIF is created for 1st activation. with the following note:

• on main branch: when deactivate avif, we will automatically generate missing webP for existing image => on PR we won't automatically create webP for existing image in this case => in this case the generate missing next-gen will keep loading but nothing will be generated
  @piotrbak what do you think? In this case, either we need to automatically generate webp for existing images when deactivate avif or not automatically clicking generate missing next-gen btn.

[piotrbak]

@Mai-Saad This behaviour is acceptable for the initial 2.2

[piotrbak]

@Mai-Saad @wordpressfan I can see that the PR is still a draft/in-progress. Where are we with this one?

[wordpressfan]

I think we are fine here based on the comments above but I requested a review from @Mai-Saad @Tabrisrp and @MathieuLamiot to add their review.