fix migration down product warehouses

internal/database/migrations/20251203083339_add_project_flock_kandang_to_product_warehouses.down.sql

@@ -20,7 +20,7 @@ ALTER TABLE product_warehouses
 
 -- Restore audit/soft-delete columns
 ALTER TABLE product_warehouses
-    ADD COLUMN IF NOT EXISTS created_by BIGINT NOT NULL REFERENCES users (id),
+    ADD COLUMN IF NOT EXISTS created_by BIGINT REFERENCES users (id),
     ADD COLUMN IF NOT EXISTS created_at TIMESTAMPTZ DEFAULT NOW(),
     ADD COLUMN IF NOT EXISTS updated_at TIMESTAMPTZ DEFAULT NOW(),
     ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;