chore: fix interfaces for product module (#5387)

Some changes based on @shahednasser's PR - https://github.com/medusajs/medusa/pull/5341

packages/product/src/models/product-option.ts

@@ -1,3 +1,4 @@
+import { DAL } from "@medusajs/types"
 import { DALUtils, generateEntityId } from "@medusajs/utils"
 import {
   BeforeCreate,
@@ -14,7 +15,6 @@ import {
 } from "@mikro-orm/core"
 import { Product } from "./index"
 import ProductOptionValue from "./product-option-value"
-import { DAL } from "@medusajs/types"
 
 type OptionalRelations =
   | "values"
@@ -39,8 +39,9 @@ class ProductOption {
   @ManyToOne(() => Product, {
     index: "IDX_product_option_product_id",
     fieldName: "product_id",
+    nullable: true,
   })
-  product: Product
+  product!: Product
 
   @OneToMany(() => ProductOptionValue, (value) => value.option, {
     cascade: [Cascade.REMOVE, "soft-remove" as any],