TypeError null allowed where string required in argument 3 ($subject) of str_replace

[joejoe04]

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

• Made sure you’re on the latest version (tested on 3.16.4, but the code is same as latest version)
• Used the search feature to ensure that the bug hasn’t been reported before ✅

Describe the bug
The customer reports the following error is logged:

PHP Fatal error: Uncaught TypeError: str_replace(): Argument #3 ($subject) must be of type array|string, null given in /www/gjcyclingshop_242/public/wp-content/plugins/wp-rocket/inc/Engine/Optimization/RegexTrait.php:151

To Reproduce
Steps to reproduce the behavior:
I'm not able to reproduce the error with WP_DEBUG enabled. However, we can see here in this part of the code:

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

Lines 146 to 149 in 9d4f8ca

	protected function restore_html_comments( $html ) {
	if ( empty( $this->html_replace ) ) {
	return $html;
	}

We need to ensure that $html is a non-empty string to prevent errors like this.

Expected behavior
We should run a check to make sure $html is a string and just return if not.

Maybe this line:

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

Line 147 in 9d4f8ca

if ( empty( $this->html_replace ) ) {

Could become this:
if ( !is_string( $html ) || empty( $this->html_replace ) ) {

Additional context
Ticket: https://secure.helpscout.net/conversation/2710047950/512718/
Website: https://www.gjcyclingshop.be

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

[hanna-meda]

For testing:

• Execute Test Plan - https://wpmediaqa.testrail.io/index.php?/runs/view/961
• Run lcp & llcssbg on PR at e2e