Allow null values to be cached with @CacheResult

Even though the JSR-107 spec forbids to store null values, our cache
abstraction allows that behaviour with a special handled (and this is
the default behaviour).

While this was working fine with our own set of annotations, the
JSR-107 interceptor counterpart was interpreting the spec sensu strictu.

We now allow for that special case as well.

Issue: SPR-13641

spring-aspects/src/test/java/org/springframework/cache/config/AnnotatedClassCacheableService.java

@@ -38,6 +38,11 @@ public class AnnotatedClassCacheableService implements CacheableService<Object>
 		return counter.getAndIncrement();
 	}
 
+	@Override
+	public Object cacheNull(Object arg1) {
+		return null;
+	}
+
 	@Override
 	public Object conditional(int field) {
 		return null;

spring-aspects/src/test/java/org/springframework/cache/config/AnnotatedJCacheableService.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -58,6 +58,12 @@ public class AnnotatedJCacheableService implements JCacheableService<Long> {
 		return counter.getAndIncrement();
 	}
 
+	@Override
+	@CacheResult
+	public Long cacheNull(String id) {
+		return null;
+	}
+
 	@Override
 	@CacheResult(exceptionCacheName = "exception", nonCachedExceptions = NullPointerException.class)
 	public Long cacheWithException(@CacheKey String id, boolean matchFilter) {

spring-aspects/src/test/java/org/springframework/cache/config/CacheableService.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,8 @@ public interface CacheableService<T> {
 
 	T cache(Object arg1);
 
+	T cacheNull(Object arg1);
+
 	void invalidate(Object arg1);
 
 	void evictEarly(Object arg1);

spring-aspects/src/test/java/org/springframework/cache/config/DefaultCacheableService.java

@@ -40,6 +40,12 @@ public class DefaultCacheableService implements CacheableService<Long> {
 		return counter.getAndIncrement();
 	}
 
+	@Override
+	@Cacheable("testCache")
+	public Long cacheNull(Object arg1) {
+		return null;
+	}
+
 	@Override
 	@CacheEvict("testCache")
 	public void invalidate(Object arg1) {