feat: Pricing update and refactor product API tests to not rely on internals (#6892)

This PR achieves the following
- Remove dependency on internals for seeding the tests (more work left, but major work done)
- Adds the workflow for updating variant's price

I will do a follow-up PR to further clean up the tests and remove all internal dependencies

packages/medusa/src/api-v2/admin/products/helpers.ts

@@ -47,6 +47,10 @@ export const remapProduct = (p: ProductDTO) => {
 }
 
 export const remapVariant = (v: ProductVariantDTO) => {
+  if (!v) {
+    return v
+  }
+
   return {
     ...v,
     prices: (v as any).price_set?.prices?.map((price) => ({

packages/medusa/src/api-v2/admin/products/validators.ts

@@ -646,7 +646,8 @@ export class AdminPostProductsProductOptionsOptionReq {
 // eslint-disable-next-line max-len
 export class ProductVariantReq extends AdminPostProductsProductVariantsVariantReq {
   @IsString()
-  id: string
+  @IsOptional()
+  id?: string
 }
 
 export class ProductTagReq {