title: Why does `map2_chr` sometimes return a `character(0)`?
body: <p>I need to be able to use the keys in a list. With <code>purrr</code>, I use the <code>map2</code> family (here <code>map2_chr</code>) and use <code>names(.)</code> to get the keys.
  However, when the original list is only 1 element long, and has no key, <code>map2_chr</code> sends back an empty vector. </p>
  
  <p>I've opened an issue on the <code>purrr</code> repo, as I think this might be a bug. </p>
  
  <pre class="lang-r prettyprint-override"><code>library(purrr)
  alist &lt;- list("bla") 
  alist %&gt;% 
    map_chr(~ .)
  #&gt; [1] "bla"
  alist %&gt;% 
    map2_chr(.y = NULL, ~ .x)
  #&gt; character(0)
  alist %&gt;% 
    map2_chr(.y = alist, ~ .x)
  #&gt; [1] "bla"
  </code></pre>
  
  <p>I'm looking for a way to get <code>"bla"</code> event when the value of <code>.y</code> is <code>NULL</code>.</p>