fix(webshipper): only add invoices if invoice generator produces a file (#1749)

packages/medusa-fulfillment-webshipper/src/services/webshipper-fulfillment.js

@@ -287,6 +287,7 @@ class WebshipperFulfillmentService extends FulfillmentService {
           fulfillmentItems
         )
 
+        if (base64Invoice) {
           invoice = await this.client_.documents
             .create({
               type: "documents",
@@ -300,6 +301,7 @@ class WebshipperFulfillmentService extends FulfillmentService {
             .catch((err) => {
               throw err
             })
+        }
 
         const cooCountries = this.options_.coo_countries
         if (