Fix error when trying to add HTML block

Problem/Motivation

Error when trying to add HTML block
Error: Unsupported operand types in vardoc_book_editor_js_settings_alter() (line 19 of profiles/vardoc/modules/vardoc_features/vardoc_book/vardoc_book.module).

Steps to reproduce

When trying to add custom HTML block from (/block/add/varbase_html_code_block)
I got this error:
Error: Unsupported operand types in vardoc_book_editor_js_settings_alter() (line 19 of profiles/vardoc/modules/vardoc_features/vardoc_book/vardoc_book.module).

Proposed resolution

In vardoc_book.module
$settings['editor']['formats']['full_html']['editorSettings'] += $extra_editor_settings;
must check if full_html format is exist