Fix a bug in the MigrationLookup migration.

The migrations with ID were being skipped by the MigrationLookup plugin.

More info here: https://www.drupal.org/project/drupal/issues/2751825

web/modules/custom/agaric_migration/migrations/agaric_comment.yml

@@ -9,7 +9,7 @@ source:
 process:
   pid:
     -
-      plugin: migration_lookup
+      plugin: migration_lookup_with_zero
       migration: agaric_comment
       source: pid
       no_stub: true
@@ -37,7 +37,7 @@ process:
   subject: subject
   uid:
     -
-      plugin: migration_lookup
+      plugin: migration_lookup_with_zero
       migration: agaric_user
       source: uid
       no_stub: true

web/modules/custom/agaric_migration/src/Plugin/migrate/process/MigrationLookupWithZero.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\agaric_migration\Plugin\migrate\process;
+
+use Drupal\migrate\MigrateSkipProcessException;
+use Drupal\migrate\Plugin\migrate\process\MigrationLookup;
+
+/**
+ * Looks up the value of a property based on a previous migration.
+ *
+ * This plugin just alters the MigrationLookup::skipOnEmpty to consider the
+ * number zero as a valid ID.
+ *
+ * @MigrateProcessPlugin(
+ *   id = "migration_lookup_with_zero"
+ * )
+ */
+class MigrationLookupWithZero extends MigrationLookup {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function skipOnEmpty(array $value) {
+    if (!array_filter($value, [$this, 'isNotEmpty'])) {
+      throw new MigrateSkipProcessException();
+    }
+  }
+
+  /**
+   * Determines if the value is not empty.
+   *
+   * The only values considered empty are: NULL, FALSE and an empty string.
+   *
+   * @param string $value
+   *   The value to test if is empty.
+   *
+   * @return bool
+   *   Return true if the value is not empty.
+   */
+  protected function isNotEmpty($value) {
+    return ($value !== NULL && $value !== FALSE && $value !== '');
+  }
+
+}