Scan should not throw on empty sequences

[GoogleCodeExporter]

The Scan<T> implementation throws when an empty sequence is found. I believe 
the correct behavior should be to yield break in that case.

This would mean a different behavior from the Scan<T, TState> implementation, 
but it is already different. Without a seed, an IEnumerable of size N will 
yield an IEnumerable of the same size, while the version with seed adds an 
extra element.

So making a 0-length IEnumerable result in a 0-length IEnumerable seems like 
the right behavior to me.

Original issue reported on code.google.com by [email protected] on 30 Dec 2014 at 1:15