1st activation of avif won't run automatically in the background

[Mai-Saad]

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Describe the bug
When we enable AVIF for the 1st time after fresh activation, AVIF won't be automatically generated (reactivate avif will make the creation works automatically)

To Reproduce
Steps to reproduce the behavior:

1. install and activate imagify
2. upload some images (small, medium, big)
3. activate avif => (notice that bulk notice isnot displayed)
4. check media library => generate next-gen link displayed (nothing generated in background)

Expected behavior
Automatic creation of avif works even when activating the option for the 1st time

Screenshots
Video and sample images
screen-capture (84).webm

Additional context
Add any other context about the problem here.

• Reactivate AVIF will work

Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming

[MathieuLamiot]

We declare maybe_generate_missing_nextgen as a callback to update_option_imagify_settings. However, update_option_xxx hooks seem to only run when updating an option, not creating it. This is explained here and here, and can be seen in WP Core code.

I think we should declare maybe_generate_missing_nextgen as a callback to add_option_imagify_settings as well

[MathieuLamiot]

Also, don't know if it is related:
@Tabrisrp I am not sure to understand the meaning of this line in maybe_generate_missing_nextgen

if ( isset( $old_value['convert_to_avif'] ) && isset( $value['convert_to_avif'] ) ) {
    return;
}

Don't we risk to bail out when both are set, but old value is set to 0, and new value to 1?

[MathieuLamiot]

discussing here