Passing request headers to string functions causes deprecations in PHP 8.1

Problem/Motivation

PHP 8.1 built-in string functions will trigger a deprecation error when a NULL is passed in. We often do this when getting something from the request headers.

Steps to reproduce

See test runs in #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves)

Proposed resolution

Provide a better default value - ie. an empty string.

Remaining tasks

User interface changes

N/a

API changes

None

Data model changes

N/a

Release notes snippet

N/a