Message list can be empty

Fixes an empty message list, which throws the following simplexml error:

simplexml_load_string(): Entity: line 2: parser error : Start tag expected, '<' not found [line 41 of /zendserver-phing/vendor/zendserverapi/zendserverapi/library/ZendService/ZendServerAPI/Adapter/MessageList.php]
simplexml_load_string():  [line 41 of /zendserver-phing/vendor/zendserverapi/zendserverapi/library/ZendService/ZendServerAPI/Adapter/MessageList.php]
simplexml_load_string(): ^ [line 41 of /zendserver-phing/vendor/zendserverapi/zendserverapi/library/ZendService/ZendServerAPI/Adapter/MessageList.php]

Change-Id: I912f57530ff7fc4f25cd30ac9c5144005419cad2