title: Sort nil to the end of an array of optional strings
body: <p>If I have an array of optional strings, and I want to sort it in ascending order with nils at the beginning, I can do it easily in a single line:</p>
  
  <pre><code>["b", nil, "a"].sorted{ $0 ?? "" &lt; $1 ?? "" } // [nil, "a", "b"]
  </code></pre>
  
  <p>But there doesn't seem to be any similarly easy solution to sort nils to the <strong>end</strong> of the array. It can be easily done with most other simple data types, for instance:</p>
  
  <pre><code>[2, nil, 1].sorted{ $0 ?? Int.max &lt; $1 ?? Int.max } // [1, 2, nil]
  </code></pre>
  
  <p>For doubles you can do the same with <code>greatestFiniteMagnitude</code>, for dates you can use <code>distantFuture</code>.  Is there any kind of equivalent for strings, or any other concise way of doing this so I can avoid writing a bunch of messy conditionals?</p>