Your if sentence is wrong, change to:

if s == ("") || s == (nil)

EDIT: Brackets are not necessary.

The error means nil has no a method split, I think perhaps you want use split with a string value, but your code generate some invalid value(nil) , and then call split, and then post the error. isn't it?

EDIT: I've read the description, it said:

you should validate that the given string is not empty or null. If so, the method should return:
"Invalid String!"

If you forgot to validate the type of the argument?

A small suggestion is you should tell author that you are using which language ;-)