[PHP8.1] Deprecated function: str_replace(): Passing null to parameter #2 ($replace) of type array|string is deprecated in Drupal\Core\Access\RouteProcessorCsrf->processOutbound()

Problem/Motivation

Using Flag on PHP 8.1 produces the following PHP deprecation message:

Deprecated function: str_replace(): Passing null to parameter #2 ($replace) of type array|string is deprecated in Drupal\Core\Access\RouteProcessorCsrf->processOutbound()

Proposed resolution

Fix the deprecation message.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.