Issue #3110064 by idebr, chandrashekhar_srijan, Vidushi Mehta, ckng, benjifisher, mikelutz: Migrate empty, <nolink> and <none> link field

core/modules/link/src/Plugin/migrate/process/FieldLink.php

@@ -60,10 +60,15 @@ class FieldLink extends ProcessPluginBase {
    */
   protected function canonicalizeUri($uri) {
     // If we already have a scheme, we're fine.
-    if (empty($uri) || parse_url($uri, PHP_URL_SCHEME)) {
+    if (parse_url($uri, PHP_URL_SCHEME)) {
       return $uri;
     }
 
+    // Empty URI and non-links are allowed.
+    if (empty($uri) || in_array($uri, ['<nolink>', '<none>'])) {
+      return 'route:<nolink>';
+    }
+
     // Remove the <front> component of the URL.
     if (strpos($uri, '<front>') === 0) {
       $uri = substr($uri, strlen('<front>'));

core/modules/link/tests/src/Unit/Plugin/migrate/process/FieldLinkTest.php

@@ -86,6 +86,18 @@ class FieldLinkTest extends UnitTestCase {
         'http://www.example.com/page#links',
         'http://www.example.com/page#links',
       ],
+      'empty' => [
+        '',
+        'route:<nolink>',
+      ],
+      'No link' => [
+        '<nolink>',
+        'route:<nolink>',
+      ],
+      'none' => [
+        '<none>',
+        'route:<nolink>',
+      ],
     ];
   }