Guard our code when the rocket_defer_inline_exclusions filter used by 3rd parties

[NataliaDrause]

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• Made sure you’re on the latest version -yes
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Describe the bug
3rd party plugins use our filter rocket_defer_inline_exclusions and pass a string instead of an array. This leads to a PHP warning in the debug.log:
PHP Warning: Invalid argument supplied for foreach() in /home4/nataliad/testsite.nataliadrause.com/wp-content/plugins/wp-rocket/inc/Engine/Optimization/DeferJS/DeferJS.php on line 140

Such errors were caused by compatibility codes in ShortPixel AI and Smart Slider 3 plugins so far.

To Reproduce
Steps to reproduce the behavior:

1. Install and activate ShortPixel AI plugin https://wordpress.org/plugins/shortpixel-adaptive-images/
2. Enable Load JavaScript Deferred option (with jQuery not being excluded from deferring)
3. Clear and preload cache.
4. The following error will be logged in debug.log:

09-Feb-2021 11:22:23 UTC] PHP Notice: Array to string conversion in .../wp-content/plugins/shortpixel-adaptive-images/includes/controllers/short-pixel-ai.class.php on line 1186
[09-Feb-2021 11:22:23 UTC] PHP Warning: Invalid argument supplied for foreach() in .../wp-content/plugins/wp-rocket/inc/Engine/Optimization/DeferJS/DeferJS.php on line 140

Expected behavior
There should be no errors.

Additional context
Code in ShortPixel AI:

public function wp_rocket_no_defer_spai_settings($regex) {
return $regex . '|spai_settings';
}

Comment by @engahmeds3ed:

I think we need to guard it and make it backward compatible so we will detect if it's string we will change it to array by exploding using | needle

Slack convo: https://wp-media.slack.com/archives/C43T1AYMQ/p1612871496110000
Ticket: https://secure.helpscout.net/conversation/1412614288/235953/

Related issue with Smart Slider 3:
Slack: https://wp-media.slack.com/archives/C60QX4FC1/p1612456408103600
Ticket: https://secure.helpscout.net/conversation/1414913336/236541/

Backlog Grooming (for WP Media dev team use only)

• Reproduce the problem
• Identify the root cause
• Scope a solution
• Estimate the effort

[engahmeds3ed]

Grooming

• Reproduce the problem
  Yes I saw the issue on customer site.

• Identify the root cause
  At 3.8 we removed safe mode for jquery option on this PR Closes #3303 Remove defer JS safe mode option #3366 and there we used the filter rocket_defer_inline_exclusions and pass it a string and then on the next minor release (3.8.1) we changed it's type to be array on this PR Fix JavaScript deferring breaks Smart Slider 3 #3424
  so if there is a code is using our 3.8 approach it will fail with our newer versions after 3.8

• Scope a solution
  We need to guard our filter value here
  

  wp-rocket/inc/Engine/Optimization/DeferJS/DeferJS.php

  Line 136 in b5edbea

  $inline_exclusions_list = apply_filters( 'rocket_defer_inline_exclusions', $this->inline_exclusions );

  
  so here we will check the type of it, if string we will explode it with the delimiter | and then force casting it to be array.

• Estimate the effort
  [XS] it will need adding new test case to pass different types of data to this filter.