[Style] Style::parseStylesFormValue() fails on lines without a pipe

Steps to reproduce:

1. Open configuration page for some text format, i.e. Full HTML
2. Drag and drop Style button to active toolbar
3. Select Style tab under CKEditor5 plugin settings
4. Put wrong value to the Styles textarea, i.e. "example".
5. Sumbit the form

The following error message appears.

Warning: Undefined array key 1 in Drupal\ckeditor5\Plugin\CKEditor5Plugin\Style::parseStylesFormValue() (line 117 of core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/Style.php).