fix(medusa,medusa-fulfillment-webshipper): Item to Webshipper line mapping (#3879)

* fix(medusa): Add missing variant relation on items

* fix: Webshipper fulfillment service

* Create kind-singers-travel.md

* Only add props if present

* add tests

.changeset/kind-singers-travel.md

@@ -0,0 +1,6 @@
+---
+"medusa-fulfillment-webshipper": patch
+"@medusajs/medusa": patch
+---
+
+fix(medusa,medusa-fulfillment-webshipper): Add missing variant + product relation on items