Generate Missing WebP progress bar can be -ve in specific case

[Mai-Saad]

Before submitting an issue please check that you’ve completed the following steps:

• Made sure you’re on the latest version => 2.1.3
• Used the search feature to ensure that the bug hasn’t been reported before

Describe the bug Generation of missing webp can be -ve if we uploaded images while generation is loading

To Reproduce Steps to reproduce the behavior:

1. Install and activate imagify
2. click generate missing webp images btn
3. make sure that Auto-Optimize images on upload is checked
4. upload some images
5. notice the progress bar of generating missing webP images

Expected behavior The progress bar is +ve, showing progress and moving

Screenshots If applicable, add screenshots to help explain your problem.

Additional context Add any other context about the problem here.

• before uploading images, the progress bar kept loading without any updates i.e 0/23 as if it isnot working

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

[Mai-Saad]

for test: when fixed, we need to check this https://github.com/wp-media/imagify-plugin/pull/820#issuecomment-1973004389