Critical error thrown when creating a new revision of an entity

Problem/Motivation

I have an entity type with a file_url field. I create a new entity of said type and I fill the file_url field with a remote url. I save the entity, edit it and save it again creating a new revision. A critical error is thrown:

Drupal\Core\Entity\EntityStorageException: SQLSTATE[22007]: Invalid datetime format: 1366 Incorrect integer value: 'http://www.example.com' for column 'fid' at row 1: INSERT INTO {file_usage}

This happens because the \Drupal\file_url\Plugin\Field\FieldType\FileUrlFieldItemList class adds file usages on entity updates when a new revision is created, but it doesn't check if the file is a remote one.

Proposed resolution

Add a check to see if the file is remote, like in the other portions of the class.

Remaining tasks

Tests.

User interface changes

None.

API changes

None.

Data model changes

None.