Question

it seems this code is valid. I am wondering what if getTest(1)

    IEnumerable<int> getTest(int n)
    {
        if (n == 0)
            yield return 1;
        else
            ;
    }

Answer 1

It will return an empty but non-null IEnumberable<int>.

Once you’ve marked a method as an iterator (by using yield anywhere in the method), it can execute zero or more yield returns to return data in the sequence.

In fact, the simplest way to get an empty IEnumberable is IEnumerable<T> Empty() { yield break; }.
Or just call Enumerable.Empty<T>().