fix(new_window): window_shell / start_directory shoud be str type not None

Author: rockandska

libtmux/session.py

@@ -198,10 +198,10 @@ def rename_session(self, new_name: str) -> "Session":
     def new_window(
         self,
         window_name: t.Optional[str] = None,
-        start_directory: None = None,
+        start_directory: str = "",
         attach: bool = True,
         window_index: str = "",
-        window_shell: None = None,
+        window_shell: str = "",
     ) -> Window:
         """
         Return :class:`Window` from ``$ tmux new-window``.