code: public static <T> Mono<T> fromCallable(Callable<? extends T> supplier) {
  		return onAssembly(new MonoCallable<>(supplier));
  	}
docs: Create a {@link Mono} producing its value using the provided {@link Callable}. If
  the Callable resolves to {@code null}, the resulting Mono completes empty.
  
  <p>
  <img class="marble" src="doc-files/marbles/fromCallable.svg" alt="">
  <p>
  @param supplier {@link Callable} that will produce the value
  @param <T> type of the expected value
  
  @return A {@link Mono}.