Fix migration issue for link fields with ‘External links only’ enabled

Problem/Motivation

Links that are intended to be external-only are currently being processed as internal links, causing issues where they can’t be saved due to validation when they contain <nolink> when migrated from empty. This affects migrations using the link field type.

Steps to reproduce

1. Create a link field with “External links only” enabled in its field settings.
2. Attempt to migrate data into this field using FieldLink as a process plugin.
3. Observe that certain values (e.g., route:) cause issues and prevent saving.

Proposed resolution

We may want to add a 'allow empty' option or something

— @mikelutz

Workaround used in the wild:
Extend the existing FieldLink migration process plugin to handle cases where external-only links are restricted by field settings. A possible solution is to override canonicalizeUri() to return an empty string for unsupported internal routes like <nolink>:

use Drupal\link\Plugin\migrate\process\FieldLink as DrupalFieldLink;
use Drupal\migrate\Attribute\MigrateProcess;

#[MigrateProcess('my_field_link')]
final class FieldLink extends DrupalFieldLink {

  protected function canonicalizeUri($uri) {
    $uri = parent::canonicalizeUri($uri);
    if ($uri === 'route:<nolink>') {
      return '';
    }

    return $uri;
  }
}

Remaining tasks

• Investigate how to check if “External links only” is enabled in field settings.
• Determine if this change should be applied conditionally based on field settings.
• Test the modified process plugin to confirm that migrations proceed correctly.

Release notes snippet

Migration of link fields with “External links only” enabled now correctly handles unsupported internal routes such as , preventing issues with saving migrated data.