Closes #822: The filter to enable webP along with AVIF is not working

[jeawhanlee]

Description

Fixes the PHP Warning thrown when Webp is enabled via filter. Fixes #822

Type of change

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

Checklists

Feature validation

• [x] I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• [x] I wrote self-explanatory code about what it does.
• [x] I named variables and functions explicitely.
• [x] I did not introduce unecessary complexity.

[Mai-Saad]

@jeawhanlee Thanks for the PR. Filter is working now, with the following notes

• Can see this when upload image while AVIF is enabled and filter set to webP (happened once) [28-Feb-2024 13:46:42 UTC] PHP Deprecated: strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated in /home/mai/Local Sites/imagifynginx6/app/public/wp-content/plugins/imagify-plugin/inc/functions/admin.php on line 415
• If the filter is set to webP while AVIF is enabled, existing images with avif , won't have generate next-gen link to generate the webp => will add separate GH about that
• If filter is activating avif, while it isnot checked in UI, we will generate the avif