Detect custom deserialized NullValue instances

Closes gh-36727

(cherry picked from commit 916cb64581)

spring-context/src/main/java/org/springframework/cache/support/AbstractValueAdaptingCache.java

@@ -87,7 +87,7 @@ public abstract class AbstractValueAdaptingCache implements Cache {
 	 */
 	@Nullable
 	protected Object fromStoreValue(@Nullable Object storeValue) {
-		if (this.allowNullValues && storeValue == NullValue.INSTANCE) {
+		if (this.allowNullValues && storeValue instanceof NullValue) {
 			return null;
 		}
 		return storeValue;

spring-context/src/testFixtures/java/org/springframework/context/testfixture/cache/AbstractCacheTests.java

@@ -24,7 +24,9 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.beans.BeanUtils;
 import org.springframework.cache.Cache;
+import org.springframework.cache.support.NullValue;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -72,6 +74,12 @@ public abstract class AbstractCacheTests<T extends Cache> {
 		assertThat(cache.get(key).get()).isNull();
 		assertThat(cache.get(key, String.class)).isNull();
 		assertThat(cache.get(key, Object.class)).isNull();
+
+		cache.put(key, BeanUtils.instantiateClass(NullValue.class));
+		assertThat(cache.get(key)).isNotNull();
+		assertThat(cache.get(key).get()).isNull();
+		assertThat(cache.get(key, String.class)).isNull();
+		assertThat(cache.get(key, Object.class)).isNull();
 	}
 
 	@Test