[outtmpl] Do not traverse `None`

Closes #1585

test/test_YoutubeDL.py

@@ -737,6 +737,7 @@ class TestYoutubeDL(unittest.TestCase):
         test(NA_TEST_OUTTMPL, 'NA-NA-def-1234.mp4')
         test(NA_TEST_OUTTMPL, 'none-none-def-1234.mp4', outtmpl_na_placeholder='none')
         test(NA_TEST_OUTTMPL, '--def-1234.mp4', outtmpl_na_placeholder='')
+        test('%(non_existent.0)s', 'NA')
 
         # String formatting
         FMT_TEST_OUTTMPL = '%%(height)%s.%%(ext)s'

yt_dlp/utils.py

@@ -6442,10 +6442,10 @@ def traverse_obj(
 
     def _traverse_obj(obj, path, _current_depth=0):
         nonlocal depth
-        if obj is None:
-            return None
         path = tuple(variadic(path))
         for i, key in enumerate(path):
+            if obj is None:
+                return None
             if isinstance(key, (list, tuple)):
                 obj = [_traverse_obj(obj, sub_key, _current_depth) for sub_key in key]
                 key = ...