How to cache null values in caffeine cache

I'm using Caffeine cache to store data received from an external system.

LoadingCache<String, String> clientCache = Caffeine.newBuilder().expireAfterWrite(1, TimeUnit.MINUTES).build(id -> {
  System.out.println("Generating new value for " + id);
  if (id.equalsIgnoreCase("1")) {
    return null;
  }
  return new Date().toString();
});

System.out.println(clientCache.get("1"));
System.out.println(clientCache.get("1"));
System.out.println(clientCache.get("2"));
System.out.println(clientCache.get("3"));

This resulted,

Generating new value for 1
null
Generating new value for 1
null
Generating new value for 2
Wed May 20 17:11:01 IST 2020
Generating new value for 3
Wed May 20 17:11:01 IST 2020
Wed May 20 17:11:01 IST 2020

Caffeine is not saving null values in cache. How to store nulls in Caffeine?

From this issue report:

The cache does not allow storing null values. If you perform a computation and return null then this indicates the data doesn't exist and the caller receives null.

In addition, it advises:

Optional is a good technique for negative caching.