Yes, they're not the same for every language, check Python or Ruby for instance. OP used Python, and those sample tests are exhaustive. C# and others as you wrote, are rather barebones and should replicate Python's ones. Also, not all languages have None, that's why even with missing sample tests, you can assume to use "" in those.

@Chrono79 Where does one view the sample tests?

If you mean in the bottom code editor when doing the kata it gives three examples, but none of them feature one that requires an empty string.

Is there a difference between languages? I'm attempting C#, there's three tests that are rather barebones.

That depends on the language, the whole paragraph is this:

If a string contains all repeating characters, it should return an empty string ("") or None -- see sample tests.

So, you should see the sample tests about which one you should return.