Allow Cache annotations to not specify any cache name

Since Spring 4.1, a CacheResolver may be configured to customize the way
the cache(s) to use for a given cache operation are retrieved. Since a
CacheResolver implementation may not use the cache names information at
all, this attribute has been made optional.

However, a fix was still applied, preventing a Cache operation without a
cache name to be defined properly. We now allow this valid use case.

Issue: SPR-13081

spring-context/src/main/java/org/springframework/cache/annotation/SpringCacheAnnotationParser.java

@@ -235,11 +235,6 @@ public class SpringCacheAnnotationParser implements CacheAnnotationParser, Seria
 					"default cache resolver if none is set. If a cache resolver is set, the cache manager" +
 					"won't be used.");
 		}
-		if (operation.getCacheNames().isEmpty()) {
-			throw new IllegalStateException("No cache names could be detected on '" +
-					ae.toString() + "'. Make sure to set the value parameter on the annotation or " +
-					"declare a @CacheConfig at the class-level with the default cache name(s) to use.");
-		}
 	}
 
 	@Override