title: Spring cacheable - filter out empty collections using SpEL
body: <p>I wanted to know is there a way to use SpEL in order to filter out values like empty collections.</p>
  
  <p>My cache currently filters out null values:</p>
  
  <pre><code>  @Cacheable(value = "groupIdToGroupNames",unless = "#result == null")
     public Map&lt;Long, Collection&lt;String&gt;&gt; findAllBySearchCustomerKey(final long groupId) {
      return idToNameClient.findAllGroupMembersById(groupId);
     } 
  </code></pre>
  
  <p>I'm trying to find a way to filter out the groups that are of size 0 but not null.
  Is there a way of doing that by using params for @Cacheable?</p>
  
  <p>Any help would be much appreciated.</p>