Allow async loads to return null.

This makes them match what `CacheLoader` already allows for synchronous
loading and reloading via `@Nullable V load(K key)` and `@Nullable V
reload(K key, V oldValue)`.

caffeine/src/main/java/com/github/benmanes/caffeine/cache/AsyncCacheLoader.java

@@ -25,6 +25,7 @@
 import java.util.function.Function;
 
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Computes or retrieves values asynchronously based on a key, for use in populating a
@@ -62,7 +63,7 @@ public interface AsyncCacheLoader<K, V> {
    *         treated like any other {@code Exception} in all respects except that, when it is
    *         caught, the thread's interrupt status is set
    */
-  CompletableFuture<? extends V> asyncLoad(K key, Executor executor) throws Exception;
+  CompletableFuture<? extends @Nullable V> asyncLoad(K key, Executor executor) throws Exception;
 
   /**
    * Asynchronously computes or retrieves the values corresponding to {@code keys}. This method is
@@ -114,7 +115,7 @@ public interface AsyncCacheLoader<K, V> {
    *         treated like any other {@code Exception} in all respects except that, when it is
    *         caught, the thread's interrupt status is set
    */
-  default CompletableFuture<? extends V> asyncReload(
+  default CompletableFuture<? extends @Nullable V> asyncReload(
       K key, V oldValue, Executor executor) throws Exception {
     return asyncLoad(key, executor);
   }

caffeine/src/main/java/com/github/benmanes/caffeine/cache/CacheLoader.java

@@ -101,7 +101,7 @@ public interface CacheLoader<K, V> extends AsyncCacheLoader<K, V> {
    * @return the future value associated with {@code key}
    */
   @Override
-  default CompletableFuture<? extends V> asyncLoad(K key, Executor executor) throws Exception {
+  default CompletableFuture<? extends @Nullable V> asyncLoad(K key, Executor executor) throws Exception {
     requireNonNull(key);
     requireNonNull(executor);
     return CompletableFuture.supplyAsync(() -> {
@@ -193,7 +193,7 @@ default CompletableFuture<? extends V> asyncLoad(K key, Executor executor) throw
    *         {@code null} if the mapping is to be removed
    */
   @Override
-  default CompletableFuture<? extends V> asyncReload(
+  default CompletableFuture<? extends @Nullable V> asyncReload(
       K key, V oldValue, Executor executor) throws Exception {
     requireNonNull(key);
     requireNonNull(executor);