Any workaround for Mono.just(null)

Question

I'm in a situation where 3rd party library returns me

Mono<Predicate<Integer>>>

Inside library there is a map of

private Map<String, Mono<Predicate<Integer>>> predicateCache

Sometimes it returns me null inside Mono and I am getting an error which I can't handle.

predicateCache.get("someKey")
  .map(predicate -> doSomething(predicate))

Is there any workaround to handle null values? use some default value or just do some error handling externally.

So how to handle similar scenarios when you don't have control over Mono.just(null) to change it at least Mono.justOrEmpty(). Because neither of below error handlings are helping

    Mono.just((String) null)
        .doOnError(ex -> System.out.println("handle error"))
        .onErrorResume(ex -> Mono.empty())
        .map(s -> doSomething(s))
        .doOnError(ex -> System.out.println("handle error"))
        .block();