title: Pandas- Concatenating two columns of string lists
body: <p>I've got an.. interesting data frame that comes from a database.  The data frame has two columns, which are lists of strings.  I need to concat the values in these two lists, to create a new column of lists.  For example:</p>
  
  <pre><code>data = [ 
      {'id': 1, 'l1': ['Luke', 'Han'], 'l2': ['Skywalker', 'Solo']}, 
      {'id': 2, 'l1': ['Darth', 'Kylo'], 'l2': ['Vader', 'Ren']},
      {'id': 3, 'l1': [], 'l2': []}
  ]                                                                                                                                                                
  df = pd.DataFrame(data)   
  </code></pre>
  
  <p>Notice the third row has no values.  You can also assume that <code>l1</code> and <code>l2</code> are of the same length.
  And I need to concat the values in l1 and l2 (with a space between), e.g.:</p>
  
  <pre><code>result = [
      {'id': 1, 'name': ['Luke Skywalker', 'Han Solo']},
      {'id': 2, 'name': ['Darth Vader', 'Kylo Ren']},
      {'id': 3, 'name': []}
  ]
  result_df = pd.DataFrame(result)
  </code></pre>