[FIX] base_import: headerless import

- Import a CSV file without header
- Untick the option 'The first row contains the label of the column'

An error is reported.

If no header, the method `_match_headers` should return consistent
object types.

opw-769117

addons/base_import/models/base_import.py

@@ -453,7 +453,7 @@ class Import(models.TransientModel):
             :rtype: (None, None) | (list(str), dict(int: list(str)))
         """
         if not options.get('headers'):
-            return None, None
+            return [], {}
 
         headers = next(rows)
         return headers, {

addons/base_import/tests/test_base_import.py

@@ -152,7 +152,7 @@ class TestMatchHeadersMultiple(TransactionCase):
 
     def test_noheaders(self):
         self.assertEqual(
-            self.env['base_import.import']._match_headers([], [], {}), (None, None)
+            self.env['base_import.import']._match_headers([], [], {}), ([], {})
         )
 
     def test_nomatch(self):