Problem/Motivation
The file_managed.filesize column could be null, if the file size couldn't be determined. So we need to check that the file size is not null before passing to format_size(), which expects an integer.
The PHP 8.1 error messages are: Deprecated function: abs(): Passing null to parameter #1 ($num) of type int|float is deprecated in format_size() (line 138 of core/includes/common.inc) and Deprecated function: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\Component\Utility\Html::escape() (line 424 of core/lib/Drupal/Component/Utility/Html.php) - there are two deprecations because the null size is also passed thru htmlspecialchars() as the @count placeholder. (Hmm, having strict type checking someday in format_size() would prevent the need to pass the number thru htmlspecialchars() :)
Steps to reproduce
Create a File entity with null size (e.g. file doesn't exist or can't be statted), and use it with the table of files field formatter.