Scan<TSource>: do not throw on empty sequence
Scan<TSource>: do not throw on empty sequence

Instead, yield break and thus return an empty sequence.

ddpruitt/morelinq,morelinq/MoreLINQ,morelinq/MoreLINQ,smzinovyev/MoreLINQ,fsateler/MoreLINQ,fsateler/MoreLINQ,ddpruitt/morelinq