Creating an empty ComponentTypeSet throws IndexOutOfRangeException

Reported externally as [Internal link] , and discussed on Slack here

The following code throws an IndexOutOfRangeException:

namespace DefaultNamespace
{
    public class Repro : MonoBehaviour
    {
        private void Awake()
        {
            var empty = new FixedList128Bytes<ComponentType>();
            new ComponentTypeSet(empty);
        }
    }
} 

The issue is that ComponentTypeSet.CheckForDuplicates() assumes there is at least one element in the set. This is a bug; an empty ComponentTypeSet is pointless, but technically valid.

We should fix the bug in CheckForDuplicates(), but also add some tests to existing methods that take ComponentTypeSet and make sure they work correctly if the type set is empty. In most cases, the correct behavior would be to early-out ASAP.