I guess it is because array type is double and can't take null so it can't be empty. I may be wrong though.

Passing test that run when you hit "test" button is just first step of validation of your code, when you hit "attempt" more complex (often random) test cases are run on your code.
So most likely your code doesn't work well in some edge cases.

I wonder how this solution passed the empty array test without throwing exception?