fix(medusa): Product import - allow null product type (#2424)

**What**
- allow the product type field to be null

**How**
- in case the type is `null`, pass `undefined` as the value of type prop to product create/update services, otherwise pass an object with `{value}` which will be upserted

Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>

.changeset/grumpy-bottles-move.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+Product import - allow null product type