server(fix[tmux_bin]): use is not None guard, remove duplicate assignment

why: "if tmux_bin" is falsy-based and silently treats pathlib.Path("")
     as None. "is not None" is more explicit and matches the type
     annotation str | pathlib.Path | None.
what:
- Change guard from "if tmux_bin" to "if tmux_bin is not None"

Author: tony

src/libtmux/server.py

@@ -143,7 +143,7 @@ def __init__(
         **kwargs: t.Any,
     ) -> None:
         EnvironmentMixin.__init__(self, "-g")
-        self.tmux_bin = str(tmux_bin) if tmux_bin else None
+        self.tmux_bin = str(tmux_bin) if tmux_bin is not None else None
         self._windows: list[WindowDict] = []
         self._panes: list[PaneDict] = []