mb_convert_encoding(): Passing null to parameter

Problem/Motivation

If an entity does not have label field the value passed down to transliterate() will be null.

Deprecated function: mb_convert_encoding(): Passing null to parameter #1 ($string) of type array|string is deprecated in Drupal\Component\Transliteration\PhpTransliteration->transliterate() (line 142 of /code/web/core/lib/Drupal/Component/Transliteration/PhpTransliteration.php)

Steps to reproduce

I have an Eform submissions and as it does not have label field throw error at http://localhost/print/pdf/eform_submission/114454

Proposed resolution

string casting