Fix for export api request for campaign subscriber activity and no ac…

…tivity returned. In this case 'data.split' will fail as data is undefined.

lib/mailchimp/MailChimpExportAPI_v1_0.js

@@ -137,7 +137,13 @@ MailChimpExportAPI_v1_0.prototype.campaignSubscriberActivity = function (params,
  * it to the callback function.
  */
 MailChimpExportAPI_v1_0.prototype.campaignSubscriberActivityProcess = function (data, callback) {
-
+
+  // when api returns empty set data variable is undefined
+  if(typeof data === 'undefined'){
+    callback(null, []);
+    return;
+  }
+
 	var resultStrings = data.split("\n");
 	var resultJSON    = [];