how to skip switchIfEmpty (avoid Mono<Optional>) if flatMap returns Mono<Void>

Question

I'm implementing a GatewayFilter, which checks the cached result first before contacting HTTP API.

@Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
    // loadFromCache may return Mono.emtpy if no cached result found
    return loadFromCache(exchange.getRequest().getPath().toString())
            .flatMap(s -> {
                // if cached value exists, return directly
                ServerHttpResponse response = exchange.getResponse();
                Flux<DataBuffer> cached = Flux.just(response.bufferFactory().wrap(s.getBytes(StandardCharsets.UTF_8)));
                return response.writeWith(cached);
            }).switchIfEmpty(Mono.defer(() -> {
                // if no cached value found, calling http api to get a response
                return chain.filter(exchange);
            }));
}

The problem is switchIfEmpty also works on Mono<Void> returned from flatMap operation. One solution is to map the value from loadFromCache to Optional<String>, then do ifPresent check in flatMap. This seems unnecessary to me. Is there any other solution without using Optional? Thanks!