[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):

yt_dlp/utils/traversal.py

@@ -360,12 +360,12 @@ def subs_list_to_dict(subs: list[dict] | None = None, /, *, lang='und', ext=None
         if not url_or_none(sub.get('url')) and not sub.get('data'):
             continue
         sub_id = sub.pop('id', None)
-        if not isinstance(sub_id, str):
+        if not isinstance(sub_id, str) or not sub_id:
             if not lang:
                 continue
             sub_id = lang
         sub_ext = sub.get('ext')
-        if not isinstance(sub_ext, str):
+        if not isinstance(sub_ext, str) or not sub_ext:
             if not ext:
                 sub.pop('ext', None)
             else: