Write a function called lastName. It should take a single String argument containing a name formatted like:

• "Challen,Chuchu"
• or, "Challen, Chuchu "
• or, " Challen , Chuchu"

That is, the last name first, followed by a comma, and then the first name, but each possibly surrounding by whitespace. In all cases above you should return "Challen", the last name. If the passed String is null, you should return null. Otherwise the string will have two comma-separated parts as shown above. You may find the String methods split and trim useful.