[cleanup] Refactor __init__.py (#2570)

* Split `__init__` code into multiple functions
* Clean up validation code by grouping similar types of options
* Expose `parse_options` to third parties

yt_dlp/utils.py

@@ -3603,6 +3603,9 @@ def match_str(filter_str, dct, incomplete=False):
 
 
 def match_filter_func(filter_str):
+    if filter_str is None:
+        return None
+
     def _match_func(info_dict, *args, **kwargs):
         if match_str(filter_str, info_dict, *args, **kwargs):
             return None