fix: make prices optional param when updating a variant (#2155)

**Why**
- It should be possible to update variant props without having to send the prices array with every update

packages/medusa/src/api/routes/admin/products/update-variant.ts

@@ -310,6 +310,7 @@ export class AdminPostProductsProductVariantsVariantReq {
   metadata?: object
 
   @IsArray()
+  @IsOptional()
   @ValidateNested({ each: true })
   @Type(() => ProductVariantPricesUpdateReq)
   prices: ProductVariantPricesUpdateReq[]