JENKINS-11390 fix - Don't return null from getMsg()

If the user makes an Accurev transaction without a comment, then
AccurevTransaction::getMsg() returns null. The impact of this is
breaking plugins (e.g. email-ext) that rely on it being a non-null
value.

Per documentation on ChangeLogSet.Entry's getMsg() method, the return
value can be empty but never null.