Fix a bug that overwrites an item list when it has only one DDE client.

Hi.
Thank you for developing a nice DDE wrapper! Anyway, I have encountered with a problem that the node-dde wrapper ignores an item, i.e. overwrites empty string, when it has only one DDE client.

clients = dde.createClients({ KEBFRun: { ESZ13: ['price','volume']}});

It has only one client but with many items. In this case, it ignores the previous item list ['price','volume'] and make a new item list with an empty string. Of course, it won't work correctly. My solution is just to add a code that checks whether the 'item' variable is provided or not.