code: static <T> Mono<T> convertToMono(Callable<T> supplier) {
  		if (supplier instanceof Fuseable.ScalarCallable) {
  			Fuseable.ScalarCallable<T> scalarCallable = (Fuseable.ScalarCallable<T>) supplier;
  
  			T v;
  			try {
  				v = scalarCallable.call();
  			}
  			catch (Exception e) {
  				return Mono.error(e);
  			}
  			if (v == null) {
  				return Mono.empty();
  			}
  			return Mono.just(v);
  		}
  		return Mono.onAssembly(new MonoCallable<>(supplier));
  	}
docs: Returns the appropriate Mono instance for a known Supplier Flux.
  
  @param supplier the supplier Flux
  
  @return the mono representing that Flux