title: Fill empty cells in column with value of other columns
body: <p>I have a HC list in which every entry should have an ID, but some entries do not have an ID. I would like to fill those empty cells by combining the the first name column and the last name column.
  How would I go about this? I tried googling for fillna and the like but couldn't get it to work.
  What I want is basically this:</p>
  
  <pre><code>If hc["ID"] == "": 
      hc["ID"] = hc["First Name"] + hc["Last Name"]
  </code></pre>