Only log online_clients if there are any

at the end of a logfile.

tsstats/log.py

@@ -148,6 +148,7 @@ def parse_logs(log_glob, ident_map=None, online_dc=True):
                     filter(lambda c: c.connected, clients.values())
                 )
 
+                if online_clients:
                     logger.debug(
                         'Some clients are still connected: %s', online_clients
                     )