don't overwrite Product default values

src/Product.ts

@@ -66,7 +66,12 @@ export class Product {
       if (this.hasOwnProperty(headers[i])) {
         if (productValues[i] == "?") {
           console.log("skipping '" + headers[i] + "'")
-          this[headers[i]] = ""
+          continue
+        }
+        if (productValues[i] == "") {
+          console.log(
+            "keeping '" + headers[i] + "' default: '" + this[headers[i]] + "'"
+          )
           continue
         }
         console.log(