Description:
------------
According to the docs 'pass' is an option for the third parameter of mb_convert_encoding() (https://php.net/function.mb-convert-encoding), see https://www.php.net/manual/en/mbstring.supported-encodings.php where it says:

"Any php.ini entry which accepts an encoding name can also use the values "auto" and "pass". mbstring functions which accept an encoding name can also use the value "auto". If "pass" is set, no character encoding conversion is performed." 

But actually an Warning occurs when using 'pass' at mb_convert_encoding() from PHP 7.3, it used to work without Warning before PHP 7.3.

See https://3v4l.org/fbRiW#output

Test script:
---------------
<?php
// https://3v4l.org/fbRiW#output
echo mb_convert_encoding('a', 'UTF-8', 'pass');
?>

Expected result:
----------------
No warning should occur or the docs need to be adjusted for the 'pass' parameter. 

Actual result:
--------------
Warning: mb_convert_encoding(): Unable to create character encoding converter in ... on line ...