in case of newline at the end of userlist file

It's a habit for some developers to append a newline at the end of the file, but line 82 did not take this into consideration. And the consequence is that JupyterHub would keep restarting.

jupyterhub_config.py

@@ -78,6 +78,8 @@ with open(os.path.join(pwd, 'userlist')) as f:
         if not line:
             continue
         parts = line.split()
+        # in case of newline at the end of userlist file
+        if len(parts) >= 1:
             name = parts[0]
             whitelist.add(name)
             if len(parts) > 1 and parts[1] == 'admin':