Update MUConversation.java

fix bug:
java.lang.IllegalArgumentException: fromIndex(7) > toIndex(0) 
at java.util.SubList.<init>(AbstractList.java:624) 
at java.util.AbstractList.subList(AbstractList.java:484) 
at com.marcelmika.lims.conversation.MUConversation.toFullJSON(MUConversation.java:395)