title: pandas: aggregate to keep first non-NaN value
body: <p>I have a dataframe with <code>test_num</code> as index:</p>
  
  <pre><code>file_num    6    7
  test_num          
  79        NaN    ↑
  148         ↑  NaN
  </code></pre>
  
  <p>I need to reduce it to keep a first available direction (arrow) for any <code>file_num</code>:</p>
  
  <pre><code>           direction
  test_num          
  79          ↑
  148         ↑
  </code></pre>
  
  <p>I've tried this:</p>
  
  <pre><code>fd.agg(lambda x: [a for a in x if a][0], axis=1)
  
  test_num
  79     NaN
  148      ↑
  
  fd.agg(lambda x: [a for a in x if a != pd.np.nan][0], axis=1)
  
  test_num
  79     NaN
  148      ↑
  </code></pre>
  
  <p>How to do what I need here?</p>