Account for undefined attachments in formatAttachment
Account for undefined attachments in formatAttachment

I was trying to load some old conversations and received an error saying could not access `map` of `undefined`. I think Facebook used to pass an undefined array in `attachments`, and this small change accounts for that.
AdamBurgess/facebook-chat-api,noodle-learns-programming/facebook-chat-api,tpphu/facebook-chat-api,ravkr/facebook-chat-api,GAMELASTER/facebook-chat-api,Schmavery/facebook-chat-api