Do not call ::getDerivativeId() on null

Problem/Motivation

I am seeing these error messages in logs:

Warning: Undefined array key "original_link" in multilingual_menu_urls_preprocess_menu() (line 126 of /.../multilingual_menu_urls/multilingual_menu_urls.module)

Error: Call to a member function getDerivativeId() on null in multilingual_menu_urls_preprocess_menu() (line 127 of /.../multilingual_menu_urls/multilingual_menu_urls.module).

Steps to reproduce

Install and use the module.

Proposed resolution

Check for empty values.

Remaining tasks

Implement.

User interface changes

None.

API changes

None.

Data model changes

None.