Fix/830 bulk optimisation

[Khadreal]

Description

Fixes #830

Documentation

User documentation

This allow users don't to bulk optimise images that aren't optimise.

Technical documentation

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

Type of change

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

New dependencies

N/A

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

• [x] I wrote self-explanatory code about what it does.
• [x] I wrote comments to explain why it does it.
• [x] I named variables and functions explicitely.
• [ ] I protected entry points against unexpected inputs.
• [x] 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.

[Miraeld]

I've tried this scenario:

1 - Upload some images, 2 - Install Imagify 3 - Enable AVIF 4 - Start a bulk optimization 5 - AVIF are generated.

👍

[Mai-Saad]

Working fine now so if no further changes we can merge it.