title: Sort rows of a dataframe in descending order of NaN counts
body: <p>I'm trying to sort the following Pandas DataFrame:</p>
  
  <pre><code>         RHS  age  height  shoe_size  weight
  0     weight  NaN     0.0        0.0     1.0
  1  shoe_size  NaN     0.0        1.0     NaN
  2  shoe_size  3.0     0.0        0.0     NaN
  3     weight  3.0     0.0        0.0     1.0
  4        age  3.0     0.0        0.0     1.0
  </code></pre>
  
  <p>in such a way that the rows with a greater number of NaNs columns are positioned first.
  More precisely, in the above df, the row with index 1 (2 Nans) should come before ther row with index 0 (1 NaN).</p>
  
  <p>What I do now is:</p>
  
  <pre><code>df.sort_values(by=['age', 'height', 'shoe_size', 'weight'], na_position="first")
  </code></pre>