[FIX] livechat: fix livechat display for website visitor

add customer_email_data in the add_message function, otherwise he would crash
later trying to get this key's value's length and nothing would be displayed
for the website visitor.

addons/im_livechat/static/src/js/im_livechat.js

@@ -196,6 +196,7 @@ var LivechatButton = Widget.extend({
             date: moment(time.str_to_datetime(data.date)),
             is_needaction: false,
             is_note: data.is_note,
+            customer_email_data: []
         };
 
         // Compute displayed author name or email