title: Test for empty nested array
body: <p>I have the following array:</p>
  
  <pre><code>array(4) {
    [29] =&gt; NULL
    [31] =&gt; NULL
    [33] =&gt; NULL
    [35] =&gt; NULL
  }
  </code></pre>
  
  <p>I would like to test all keys if all keys contain NULL values. </p>