file_requirements() can trigger a PHP 8.1 deprecation notice when called without a SERVER_SOFTWARE server env variable

Problem/Motivation

Discovered in #3277649: Deprication notice for htmlspecialchars with php 8.1, which calls file_requirements() from a kernel test.

Steps to reproduce

Proposed resolution

Add a ?? '' after \Drupal::request()->server->get('SERVER_SOFTWARE')

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet