PHP 8.1 Deprecated function: rtrim(): Passing null to parameter

[srishtis]

Deprecated - rtrim(): Passing null to parameter #1 ($string) of type string is deprecated

used in
wp-includes/formatting.php:2809
rtrim()
wp-includes/formatting.php:2809
untrailingslashit()
wp-includes/formatting.php:2794
trailingslashit()
wp-content/mu-plugins/http-concat/concat-utils.php:27
WPCOM_Concat_Utils::realpath()
wp-content/mu-plugins/http-concat/cssconcat.php:84
WPcom_CSS_Concat->do_items()
wp-includes/functions.wp-styles.php:68
wp_print_styles()
wp-includes/class-wp-hook.php:308
do_action('wp_head')
wp-includes/general-template.php:3049
wp_head()
wp-includes/template-canvas.php:17
MU Plugin: http-concat

[srishtis]

Also Found in WP version 6.3 and version 6.3.1