Fixes #826 Only generate missing next-gen when enabling AVIF option

[remyperona]

Description

This PR changes the action happening when enabling the AVIF option. Instead of running the bulk optimize process, it runs the generate missing next-gen process, so already optimized or generated images should not be recreated for nothing.

Fixes #826

Type of change

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

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.

Code style

• I wrote self-explanatory code about what it does.
• I named variables and functions explicitely.
• I protected entry points against unexpected inputs.
• I did not introduce unecessary complexity.

[Mai-Saad]

Working as expected