title: Error: Expression<String?> is not convertible to String
body: <p>My code is:</p>
  
  <pre><code>func getTimeStamps( tablename : String) -&gt; String {
      let time_stamps = db["time_stamps"]
      let t_tabelle = Expression&lt;String?&gt;["tabelle"]
      let t_time_stamp = Expression&lt;String?&gt;["TIME_StAMP"]
      let query = time_stamps.filter(like(tablename, t_tabelle))
  
      return query[t_time_stamp]
  }
  </code></pre>
  
  <p>But I get an error on conversion:</p>
  
  <blockquote>
    <p><code>Expression&lt;String?&gt; is not convertible to String</code></p>
  </blockquote>
  
  <p>How can I return a String?</p>
  
  <p>Thanks 
  Hauke</p>