[FIX] base_import: import subfields

To reproduce
============
- on any app try to import a record that has subfields
- subfields are not matched and not even found on the list

Problem
=======
comparing to 16 **Allowing subfields matching** is disabled here in 16.1,
because of this condition that is checking the list of fields on a variable `fields`,
but this variable is not set before importing the file, so the condition is always true.

Solution
========
apply the condition on `res.fields` instead

opw-3209365

closes odoo/odoo#115639

X-original-commit: 284fab64e33ca398a0ebf69d3d6d59685480156d
Signed-off-by: Luca Vitali <luvi@odoo.com>

addons/base_import/static/src/import_model.js

@@ -432,7 +432,7 @@ export class BaseImportModel {
             }
         }
 
-        if (!this.fields.length) {
+        if (!res.fields.length) {
             this.importOptionsValues.advanced.value = res.advanced_mode;
         }