title: Use map to return NA when element missing from nested list
body: <p>I have a nested list:</p>
  
  <pre><code>test &lt;- list(
    one = list(text = "String", number = "1"),
    two = list(text = "String"),
    three = list(text = "String", number = "3")
  )
  </code></pre>
  
  <p>And I can use <code>map_chr(test, "text")</code> to return every text value, but how would I get <code>map_chr(test, "number")</code> to work and return:</p>
  
  <pre><code>one  two  three
  "1"  NA   "3"
  </code></pre>