[core] Fix --ignore-no-formats-error (#10345)

Fixes regression in 5ce582448e

Closes #10344
Authored by: Grub4K

Co-authored-by: Simon Sawicki <contact@grub4k.xyz>

yt_dlp/utils/_utils.py

@@ -5120,6 +5120,9 @@ def __init__(self, extension, /):
 
     @classmethod
     def sanitize_extension(cls, extension, /, *, prepend=False):
+        if extension is None:
+            return None
+
         if '/' in extension or '\\' in extension:
             raise cls(extension)