List对象
unless = "#result==null or #result.size()==0"
unless = "#result==null or #result.isEmpty()"
普通对象,就够了
unless = "#result==null"
https://stackoverflow.com/questions/25977710/spring-cacheable-filter-out-empty-collections-using-spel
https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#expressions-operators-logical