Add Flux#justOrEmpty method

[He-Pin]

Motivation:
I have some cases when I'm using Flux.

    if (resp == null) {
        return Flux.empty();
    } else {
        return Flux.just(resp);
    }

There is an Stream#ofNullable, so I think this method would be helpful.

[chemicL]

Hey, thanks for the suggestion.

Mono.justOrEmpty(@Nullable T) exists since #26
You can combine it with .flux():

Object x = null;
Flux<Object> f = Mono.justOrEmpty(x).flux();

f.subscribe(System.out::println, System.err::println, () -> System.out.println("done"));

I am not sure that we need the same factory in Flux. What is stopping you from returning Mono directly in the use cases you encounter?

[He-Pin]

That's true, but sometimes, the return type is fixed to Flux , because there may be more elements. We are using this in an SSR scene, it's only one element once it fails, or I can make the return type a Publisher<T>