Fix empty session being set

save_session method relies on checking if a session is empty to return early. This fixes the boolean evaluation of a permanent but empty session evaluating as True.

Closes #193

src/flask_session/sessions.py

@@ -20,6 +20,9 @@ def total_seconds(td):
 
 class ServerSideSession(CallbackDict, SessionMixin):
     """Baseclass for server-side based sessions."""
+
+    def __bool__(self) -> bool:
+        return bool(dict(self)) and self.keys() != {"_permanent"}
 
     def __init__(self, initial=None, sid=None, permanent=None):
         def on_update(self):