title: Purrr::map_df() drops NULL rows
body: <p>When using <code>purrr::map_df()</code>, I will occasionally pass in a list of data frames where some items are <code>NULL</code>. When I do, <code>map_df()</code> returns a data frame with fewer rows than the the original list. </p>
  
  <p>I assume what's going on is that <code>map_df()</code> calls <code>dplyr::bind_rows()</code> which ignores <code>NULL</code> values. However, I'm not sure how to identify my problematic rows.</p>
  
  <p>Here's an example:</p>
  
  <pre><code>library(purrr)
  
  problemlist  &lt;- list(NULL, NULL, structure(list(bounds = structure(list(northeast = structure(list(
      lat = 41.49, lng = -71.46), .Names = c("lat", "lng"
  ), class = "data.frame", row.names = 1L), southwest = structure(list(
      lat = 41.49, lng = -71.46), .Names = c("lat", "lng"
  ), class = "data.frame", row.names = 1L)), .Names = c("northeast", 
  "southwest"), class = "data.frame", row.names = 1L), location = structure(list(
      lat = 41.49, lng = -71.46), .Names = c("lat", "lng"
  ), class = "data.frame", row.names = 1L), location_type = "ROOFTOP", 
      viewport = structure(list(northeast = structure(list(lat = 41.49, 
          lng = -71.46), .Names = c("lat", "lng"), class = "data.frame", row.names = 1L), 
          southwest = structure(list(lat = 41.49, lng = -71.46), .Names = c("lat", 
          "lng"), class = "data.frame", row.names = 1L)), .Names = c("northeast", 
      "southwest"), class = "data.frame", row.names = 1L)), .Names = c("bounds", 
  "location", "location_type", "viewport"), class = "data.frame", row.names = 1L))
  
  # what actually happens
  map_df(problemlist, 'location')
  
  #     lat    lng
  # 1 41.49 -71.46
  
  
  # desired result
  map_df_with_Null_handling(problemlist, 'location') 
  
  #     lat    lng
  # 1    NA     NA
  # 2    NA     NA
  # 3 41.49 -71.46
  </code></pre>
  
  <p>I considered wrapping my <code>location</code> accessor in one of purrr's error handling functions (eg. <code>safely()</code> or <code>possibly()</code>), but it's not that I'm running into errors--I'm just not getting the desired results. </p>
  
  <p>What's the best way to handle <code>NULL</code> values with <code>map_df()</code>?</p>