Empty sequence

[reddavis]

Adds an Empty sequence that has the option of returning immediately or never.

Empty<Int>().sink(
    receiveValue: { print($0) },
    receiveCompletion: { completion in
        switch completion {
        case .finished:
            print("Finished")
        case .failure:
            print("Failed")
        }
    }
)

// Prints:
// Finished

or

Empty<Int>(completeImmediately: false).sink(
    receiveValue: { print($0) },
    receiveCompletion: { completion in
        switch completion {
        case .finished:
            print("Finished")
        case .failure:
            print("Failed")
        }
    }
)

// Prints:
//