title: Filling in NaN columns if another column is not NaN w/ string concatenation
body: <p>I'm looking to concatenate two columns together if they are not NaN, like so: </p>
  
  <pre><code>if(df[pd.notnull([df["Col1"]])] and df[pd.notnull([df["Col2"]])]):
      df["Col3"] = df["Col1"] + df["Col2"]
  </code></pre>
  
  <p>If both of these columns are not NULL/NaN, put the other two strings together and put that into column 3.</p>
  
  <p>How might I go about doing this?
  pd.notnull does not behave how I would expect it to.</p>
  
  <p>I am hoping for it to behave like this:</p>
  
  <pre><code>"First Name" "Last Name" "Full Name"
   a            b           a b
   a1           b1          a1 b1
   a2           b2          a2 b2
  </code></pre>
  
  <p>Where, before formatting it would have NaN in the "Full Name" Column.</p>
  
  <p>This is how the data would look before formatting:</p>
  
  <pre><code>"First Name" "Last Name" "Full Name" 
  a            b            NaN
  a1           b1           NaN
  a2           b2           NaN
  NaN          NaN          a3 b3
  </code></pre>