Handling when msgContent is null instead of throwing NPE

* if msgContent is null then <tag name="Some Name"/> is printed
* if entry of composite msgContent is null then:
    <tag name="Some Name">
      <inner-tag name="Inner Name">
        Some Value
      </inner-tag>
      null  <- it's that null entry
    </tag>
Removed unnecessary SimpleMsg costructors

Make use of autoboxing instead of many constructors
Added missing SimpleMsgTest jUnit tests