title: Combine two columns, ignore empty cells and add a separator
body: <p>I need to combine column <code>1</code> and <code>2</code> in <code>3</code> with separator <code>,</code> but ignore empty cells.
  So I have this dataframe:</p>
  
  <pre><code>         1     2
  0  A, B, B     D
  1           C, D
  2  B, B, C  D, A
  </code></pre>
  
  <p>And need to create column <code>3</code> (desired output):</p>
  
  <pre><code>         1     2              3
  0  A, B, B     D     A, B, B, D
  1           C, D           C, D
  2  B, B, C  D, A  B, B, C, D, A
  </code></pre>
  
  <p>So as you see here, empty sell was ignored and <code>,</code> separate elements between in df["3"] (<code>B, B, C, D, A</code>). </p>
  
  <p>I try to do this with simple concatenation, but didn't succeed. </p>
  
  <p>If I simple concatenate <code>df["1"] + df["2"]</code> I will get that the last element of first column combine with first element of last column (<code>BD</code>, <code>CD</code>):</p>
  
  <pre><code>         1     2            3
  0  A, B, B     D     A, B, BD
  1           C, D         C, D
  2  B, B, C  D, A  B, B, CD, A
  </code></pre>
  
  <p>If add ", " (<code>df["1"] + ", " + df["2"]</code>):</p>
  
  <pre><code>         1     2              3
  0  A, B, B     D     A, B, B, D
  1           C, D         , C, D
  2  B, B, C  D, A  B, B, C, D, A
  </code></pre>
  
  <p>You will see that each empty cell replace with ", " and added to df["3"] (example = <code>, C, D</code>, but I need <code>C, D</code>).</p>
  
  <p>Code for reproduce:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({"1":["A, B, B","","B, B, C"], "2":["D","C, D","D, A"]})
  print(df)
  </code></pre>