Allow -OO mode for docstring_decorator (#29689)

Fixes
```
  File "/nix/store/rv8xdwghdad9jv2w86b8g08kan9l6ksm-python3.11-transformers-4.38.2/lib/python3.11/site-packages/transformers/models/auto/configuration_auto.py", line 987, in <module>
    class AutoConfig:
  File "/nix/store/rv8xdwghdad9jv2w86b8g08kan9l6ksm-python3.11-transformers-4.38.2/lib/python3.11/site-packages/transformers/models/auto/configuration_auto.py", line 1011, in AutoConfig
    @replace_list_option_in_docstrings()
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/rv8xdwghdad9jv2w86b8g08kan9l6ksm-python3.11-transformers-4.38.2/lib/python3.11/site-packages/transformers/models/auto/configuration_auto.py", line 966, in docstring_decorator
    lines = docstrings.split("\n")
            ^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'split'
```

src/transformers/models/auto/configuration_auto.py

@@ -990,6 +990,9 @@ def _list_model_options(indent, config_to_class=None, use_model_types=True):
 def replace_list_option_in_docstrings(config_to_class=None, use_model_types=True):
     def docstring_decorator(fn):
         docstrings = fn.__doc__
+        if docstrings is None:
+            # Example: -OO
+            return fn
         lines = docstrings.split("\n")
         i = 0
         while i < len(lines) and re.search(r"^(\s*)List options\s*$", lines[i]) is None: