title: Check if the group has NaN values
body: <p>I have a dataframe and I would like to <code>groupby</code> by <code>bq_market_id</code> and then check if there is any <code>NaN</code> values in <code>bq_back_price</code> in each group if yes then <code>True</code> per group if no then <code>False</code> per group. </p>
  
  <pre><code>bq_selection_id bq_balance  bq_market_id  bq_back_price
  0         45094462     185.04       7278437           1.97
  1         45094462     185.04       7278437           1.97
  2         45094463     185.04       7278437           3.05
  3         45094463     185.04       7278437           3.05
  4         45094464     185.04       7278437           5.80
  5         45094464     185.04       7278437           5.80
  6         45094466     185.04       7278437         200.00
  7         45094466     185.04       7278437         200.00
  8         45094465     185.04       7278437            NaN
  9         45094465     185.04       7278437            NaN
  </code></pre>
  
  <p>How do i do this? I tried the following, but it did not work.</p>
  
  <pre><code>bb.groupby('bq_market_id')['bq_back_price'].isnull().any()
  </code></pre>