PHP 8.x: fix TypeError in config hook

[mlutfy]

Overview

Using vanilla CiviCRM with the old ckeditor 4, attempting to upload a file results in a fatal TypeError on PHP 8.0 and later.

To reproduce:

• Login to https://dmaster.demo.civicrm.org/civicrm/
• Go to Mailing > New Mailing
• Click the image icon
• then click "browse server"

Before

TypeError: array_merge(): Argument #2 must be of type array, null given in array_merge() (line 1490 of /srv/buildkit/build/dmaster/web/sites/all/modules/civicrm/CRM/Utils/Hook.php).

After

no error

Technical Details

Changing the function definition to default to $flags = [] didn't seem to help, so I opted for an ugly fix.

I opened this PR against the RC, because it's a fatal PHP 8 bug.

[civibot]

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[colemanw]

IMO we could also do this for 5.67 since there's already been one regression fix merged into that branch since yesterday's release.

[totten]

Agree with @colemanw, we should also backport. Added #27998.