title: pandas dataframe multiplication with missing values
body: <p>I have a dataframe with 2columns (floating types), but one of them has missing data represented by a string ".." 
  When performing a multiplication operation, an exception is raised and the whole operation is aborted.<br>
  What I try to achieve is to perform the multiplication for the float values and leave ".." for the missing ones.  </p>
  
  <pre><code>2 *  6  
  .. * 4 
  </code></pre>
  
  <p>should give <code>[12, ..]</code><br>
  I found a naive solution consisting in replacing <code>..</code> by 0 then perform the multiplication, then replace back the 0 by <code>..</code> 
  It doesn't seem very optimized. Any other solution?  </p>
  
  <pre><code>df['x'] = pd.to_numeric(df['x'], errors='coerce').fillna(0)  
  mg['x'] = df['x'] * df["Value"]  
   for col in mg.columns:
      mg[col] = mg[col].apply(update)
  def update(v):
      if (v == 0):
          return ".."
      return v
  </code></pre>