title: rbind a list of data.frames while keeping NULL elements in R
body: <p>I would like to <code>rbind</code> a <code>list</code> of <code>data.frame</code> and transform <code>NULL</code> elements into NA using R. Consider the following example,</p>
  
  <pre><code>l &lt;- list(data.frame(C1 = 1, C2 = 2),
            NULL,
            data.frame(C1 = 3))
  
  # bind_rows results
  dplyr::bind_rows(l)
  
  #   C1 C2
  # 1  1  2
  # 2  3 NA
  
  # Desired output
  data.frame(C1 = c(1, NA, 3), C2 = c(2, NA, NA))
  
  #   C1 C2
  # 1  1  2
  # 2 NA NA
  # 3  3 NA
  </code></pre>