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-context/src/test/resources/org/springframework/cache/config/cache-advice.xml

@@ -10,6 +10,7 @@
 	<cache:advice id="cacheAdviceInterface" cache-manager="cacheManager">
 		<cache:caching cache="testCache">
 			<cache:cacheable method="cache"/>
+			<cache:cacheable method="cacheNull"/>
 			<cache:cacheable method="conditional" condition="#classField == 3"/>
 			<cache:cacheable method="unless" unless="#result > 10"/>
 			<cache:cacheable method="key" key="#p0"/>