Fix return a value that will be nullalbe on mapNotNull

[KSH-code]

reactor/reactor-kotlin-extensions#52

The above issue explains mapNotNull does not change values into non-null values.

Thus, below code occurs an error such as Operator call corresponds to a dot-qualified call 'it.times(3)' which is not allowed on a nullable receiver 'it'.

    Flux.just(1, 2, 3, null).mapNotNull {
        it
    }.map {
        println(it * 3)
    }

Therefore, mapNotNull method must return @Nonnull value only.

[chemicL]

I'm not very knowledgeable in Kotlin, but the proposed change seems to be redundant. The reactor.core.publisher package has a NonNullApi annotation which should be equivalent to what this PR introduces but for all the methods' return values. Can you please explain why this change would be required?

[KSH-code]

@chemicL In my understanding, @NonNullApi is applied to only Flux or Mono. not for a generic class T.
And also I'm lack of understanding about generic types and annotations. I will check out those then I will fix this pr.

[chemicL]

I don't think this changes anything. I had a look at the signatures of map vs mapNotNull and essentially they're the same:

public final <R> Mono<R> map(Function<? super T, ? extends R> mapper)

public final <R> Mono<R> mapNotNull(Function <? super T, ? extends R> mapper)

The it variable in your report refers to the argument of Java's Function#apply(T) method (where the type definition is Function<T, R>), which we don't control.

In order to have this work the way you envision, I think java.util.function.Function<T, R>#apply(T) would need to be annotated with @Nonnull and that I'm afraid is not happening. If I'm mistaken, please provide a concrete example (repository/zip) where applying this strategy solves a problem for a Kotlin codebase in a similar scenario using Function from the JDK and we can reopen the PR/issue and work from there. For now I'm rejecting the PR though. Thanks for taking the time and investigating - please do get back if you have evidence we can fix something here.