PHP 8.5 - Deprecated function: Using null as an array offset is deprecated, use an empty string instead

Problem/Motivation

On PHP 8.5, two deprecation warnings are triggered when visiting any entity form that has a Flag field, caused by null being used as an array offset in FlaggingStorage. This happens because $account->id() returns null in certain contexts (e.g. anonymous users), and that value is then used as a key in the static cache arrays inside loadIsFlagged() and loadIsFlaggedMultiple().

Steps to reproduce

• Install Drupal with PHP 8.5
• Install the Flag module
• Create a flag and enable it on a content type
• Visit an entity form for that content type
• Observe deprecation warnings in the logs