Only add sessions refresh data to existing sessions

Prior to change session were created for _every_ request (including API
requests).

babybuddy/middleware.py

@@ -26,12 +26,13 @@ class UserTimezoneMiddleware:
 
 class RollingSessionMiddleware:
     """
-    Periodically resets the session expiry.
+    Periodically resets the session expiry for existing sessions.
     """
     def __init__(self, get_response):
         self.get_response = get_response
 
     def __call__(self, request):
+        if request.session.keys():
             session_refresh = request.session.get('session_refresh')
             if session_refresh:
                 try: