Consider returning an empty collection instead of an exception for MaxByAsync/MinByAsync

#### Feature request

**Behavior change on an existing operator.**

> Which subcomponent library: Async.Ix

> Which next library version: major (runtime behaviour breaking change)

> Please describe the feature.

The versions of this method in the framework only throw exceptions for ValueTypes, instead preferring to return null for reference types. In the spirit of this, it may be worth returning the empty List from these methods instead of throwing an error. I'll probably put together a branch with this on later this week for consideration and our own internal use.