Fixes #721 Prevent PHP errors

[Tabrisrp]

Check if array keys exists before using them

Fixes #721

[Mai-Saad]

@Tabrisrp Thanks for the PR. Can you please check the following cases 🙏
Can reproduce the following error using PHP 8.2 and the PR

[12-Jul-2023 10:41:48 UTC] PHP Warning:  Undefined array key "option_page" in /var/www/new.rocketlabsqa.ovh/htdocs/wp-content/plugins/imagify/inc/classes/class-imagify-settings.php on line 134
[12-Jul-2023 10:41:48 UTC] PHP Deprecated:  htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in /var/www/new.rocketlabsqa.ovh/htdocs/wp-content/plugins/imagify/inc/classes/class-imagify-settings.php on line 134

Scenario1:
steps:
1- use a wrong API key
2- correct the API key and save

Scenario2: => even without step 1 , we have the same error on newlabs
1- delete AS actions table
2- fresh install and activate of the PR

[Tabrisrp]

Thanks, I updated the code to prevent it too

[piotrbak]

@engahmeds3ed Could you do a CR after last changes here?

[mostafa-hisham]

@piotrbak i didn't get the warning in the issue but got this one

[30-Aug-2023 04:53:45 UTC] PHP Deprecated:  Return type of Imagify_Files_Iterator::accept() should either be compatible with FilterIterator::accept(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/mostafa/Local Sites/wp-copra/app/public/wp-content/plugins/imagify-plugin/inc/classes/class-imagify-files-iterator.php on line 65

[30-Aug-2023 05:29:07 UTC] PHP Deprecated:  Return type of Imagify_Files_Recursive_Iterator::accept() should either be compatible with FilterIterator::accept(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/mostafa/Local Sites/wp-copra/app/public/wp-content/plugins/imagify-plugin/inc/classes/class-imagify-files-recursive-iterator.php on line 55

Steps:
php v8.1
1- add wp-content/themes/twentytwentytwo/ folder in imagify settings (this triggers the first warning)
2- click bulk optimization (this triggers a second warning after the first step)

FYI: adding #[\ReturnTypeWillChange] fix the problem but i am not sure if it is the best way to handle it.

and when i did the second testing scenario here i got alot of fatal error but i don't think it is related to this PR.
I think it should be covered here