strnatcasecmp(): Passing null to parameter #2 in LayoutPluginManager

Problem/Motivation

Deprecated function: strnatcasecmp(): Passing null to parameter #2 ($string2) of type string is deprecated in Drupal\Core\Layout\LayoutPluginManager->Drupal\Core\Layout\{closure}() (line 204 of core/lib/Drupal/Core/Layout/LayoutPluginManager.php).

Steps to reproduce

Have a layout with an empty label or category.
See #3089418: Layout selection weight sorting for another example.

See #5 and #6 for the root cause! Still the cast might make sense to mitigate the problem?

Proposed resolution

Cast the NULL value to string, like in other similar issues (see related).

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet