Fix migration to exclude null user_id in activity_user population

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');
     }