title: Combine pandas string columns with missing values
body: <p>I need to concat the strings in 2 or more columns of a pandas dataframe.</p>
  
  <p>I found this <a href="https://stackoverflow.com/questions/19377969/combine-two-columns-of-text-in-dataframe-in-pandas-python">answer</a>, which works fine if you don't have any missing value. Unfortunately, I have, and this leads to things like "ValueA; None", which is not really clean.</p>
  
  <p>Example data:</p>
  
  <pre><code>col_A  | col_B
  ------ | ------
  val_A  | val_B 
  None   | val_B 
  val_A  | None 
  None   | None
  </code></pre>
  
  <p>I need this result:</p>
  
  <pre><code>col_merge
  ---------
  val_A;val_B
  val_B
  val_A
  None
  </code></pre>