fix(medusa): Throw on flat rate shipping options with no amount (#2628)

integration-tests/api/factories/simple-product-factory.ts

@@ -129,7 +129,7 @@ export const simpleProductFactory = async (
     await simpleProductVariantFactory(connection, factoryData)
   }
 
-  return manager.findOne(
+  return await manager.findOne(
     Product,
     { id: prodId },
     { relations: ["tags", "variants", "variants.prices"] }