stubtest: correct type annotations in _Arguments (#16897)

Two fields in the `_Arguments` class, `mypy_config_file` and
`custom_typeshed_dir`, can take on a None value, but they are not marked
as such. Calling `stubtest.parse_options` on an empty list of arguments
reproduces the situation where these two fields are None.

Author: sam-xif

mypy/stubtest.py

@@ -1878,8 +1878,8 @@ class _Arguments:
     allowlist: list[str]
     generate_allowlist: bool
     ignore_unused_allowlist: bool
-    mypy_config_file: str
-    custom_typeshed_dir: str
+    mypy_config_file: str | None
+    custom_typeshed_dir: str | None
     check_typeshed: bool
     version: str
 
@@ -1922,7 +1922,7 @@ def test_stubs(args: _Arguments, use_builtins_fixtures: bool = False) -> int:
     options.incremental = False
     options.custom_typeshed_dir = args.custom_typeshed_dir
     if options.custom_typeshed_dir:
-        options.abs_custom_typeshed_dir = os.path.abspath(args.custom_typeshed_dir)
+        options.abs_custom_typeshed_dir = os.path.abspath(options.custom_typeshed_dir)
     options.config_file = args.mypy_config_file
     options.use_builtins_fixtures = use_builtins_fixtures