Fixes #4397 Guard against preg_replace() errors when moving meta charset to head

[Tabrisrp]

Description

This PR adds guards to move_meta_charset_to_head(), to always return a string in case one of the preg_replace() encounters an error and returns null instead of the expected string value.

Fixes #4397

Type of change

• Bug fix (non-breaking change which fixes an issue)

Is the solution different from the one proposed during the grooming?

No grooming done

How Has This Been Tested?

• Automated tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Existing unit tests pass locally with my changes

[Mai-Saad]

Using the page source mentioned here #4397 (comment), can see now that no 500 error in console while delay js is enabled with/without default exclusions (no errors now in debug.log as well). but can see a console error when minify/combine js is enabled (on both trunk and the PR), @Tabrisrp do we need to handle anything for that? or is that expected from the test page?
Uncaught SyntaxError: Unexpected identifier for both wp-embed.min.js and jquery-migrate.min.js

[Tabrisrp]

@Mai-Saad This is probably coming from the test page, it doesn't seem to be related to the change in this PR.

[piotrbak]

@Tabrisrp It works liek expected. I do have a question though:

1. I manipulated pcre.backtrack_limit and set it to low limit
2. Visited the page without RUCSS enabled
3. Received this error: https://snippi.com/s/5lck17t

Should anything be happening related to RUCSS when the feature is disabled?

[piotrbak]

The PR itself is fine:
testrail-report-275.pdf