[utils] subs_list_to_dict: Fix empty value handling (#17311)

Authored by: doe1080

test/test_traversal.py

@@ -546,6 +546,18 @@ class TestTraversalHelpers:
                 {'url': 'https://example.com/subs/de4'},
             ],
         }, 'non str types should be replaced by default id'
+        assert traverse_obj([
+            {'name': '', 'ext': '', 'url': 'https://example.com/subs/en'},
+        ], [..., {
+            'id': 'name',
+            'ext': 'ext',
+            'url': 'url',
+        }, all, {subs_list_to_dict(lang='en', ext='vtt')}]) == {
+            'en': [{
+                'ext': 'vtt',
+                'url': 'https://example.com/subs/en',
+            }],
+        }, 'empty id and ext should be replaced by defaults'
 
     def test_trim_str(self):
         with pytest.raises(TypeError):