Using null as an array offset is deprecated in WebformPluginElementController

Problem/Motivation

On PHP 8.5 deprecation warning is triggered in Drupal\webform\Controller\WebformPluginElementController:

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

In the formats section, getItemDefaultFormat() can return NULL, and that value is used as an array key in isset($formats[$default_format]). Using NULL as an array offset is deprecated in newer PHP versions.

Steps to reproduce

1. Install and enable Webform on a Drupal site running PHP 8.5
2. Open the page that builds the Webform element plugin listing (the code path that executes index() in the controller).
3. Ensure at least one element/plugin returns NULL from getItemDefaultFormat().
4. Check logs/output for the deprecation warning.

Proposed resolution

Normalize NULL to an empty string before using it as an array key.

Update:
$default_format = $webform_element->getItemDefaultFormat();

to:
$default_format = $webform_element->getItemDefaultFormat() ?? '';

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.