\Drupal\workspaces\WorkspaceRepository::loadTree() uses NULL as an array key

Problem/Motivation

\Drupal\workspaces\WorkspaceRepository::loadTree() uses NULL as an array key to signify that a workspace has no parent. This cannot work on PHP 8.5 as this triggers a deprecation. As it goes a NULL as an array key has always been an empty string so let's just continue with that. See https://3v4l.org/6qLRr

Steps to reproduce

Run \Drupal\Tests\workspaces\Kernel\WorkspaceEntityRepositoryTest on PHP 8.5.

Proposed resolution

Use an empty string as an array key in \Drupal\workspaces\WorkspaceRepository::loadTree() when the workspace has no parent.

Remaining tasks

User interface changes

None

Introduced terminology

N/a

API changes

None

Data model changes

None

Release notes snippet

N/a