Fix migration to exclude null user_id in activity_user population

- Added a condition to skip rows with null `user_id` during the `INSERT` operation in the migration script.

.changes/unreleased/Fixed-20251211-214908.yaml

@@ -0,0 +1,7 @@
+kind: Fixed
+body: |
+    Fix migration query after previous fix
+time: 2025-12-11T21:49:08.899926492+01:00
+custom:
+    Issue: "466"
+    SchemaChange: No schema change

src/Bundle/ChillActivityBundle/migrations/Version20251118124241.php

@@ -39,7 +39,7 @@ final class Version20251118124241 extends AbstractMigration
         $this->addSql("COMMENT ON COLUMN activity_user.by_migration IS 'For backup purpose - can be safely deleted after a while. See migration \\Chill\\Migrations\\Activity\\Version20251118124241'");
 
         $this->addSql('INSERT INTO activity_user (activity_id, user_id, by_migration)
-            SELECT id, user_id, true FROM activity
+            SELECT id, user_id, true FROM activity WHERE user_id is not null
             ON CONFLICT DO NOTHING');
     }