Steps to reproduce the issue

On PHP 8.1 with error reporting set to maximum in Global Configuration and PHP configured to log errors into a log file, edit an article in backend which has no Read More button and so has no intro text. Save the article.

Expected result

No PHP notice or warning or error logged.

Actual result

PHP notice PHP Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /home/richard/lamp/public_html/joomla-cms-4.2-dev/plugins/content/fields/fields.php on line 66.

System information (as much as possible)

PHP 8.1

Additional comments

Happens since PR #38977 has been merged.

=> ping @nikosdion