title: Get a boolean dataframe from a dataframe with true if after the first_valid_date and false otherwise
body: <p>I have a Dataframe with several NaN like this one :</p>
  <pre><code>              A    B    C
  2021-08-31  1.0  NaN  NaN
  2021-09-01  2.0  NaN  5.0
  2021-09-02  4.0  3.0  NaN
  2021-09-03  NaN  7.0  NaN
  2021-09-06  2.0  5.0  NaN
  2021-09-07  9.0  NaN  5.0
  2021-09-08  4.0  5.0  NaN
  </code></pre>
  <p>I would like to have a Dataframe that, for each column, is True if the element is the first_valid_date or after and False otherwise :</p>
  <pre><code>             A      B      C
  2021-08-31  True  False  False   
  2021-09-01  True  False  True
  2021-09-02  True  True   True
  2021-09-03  True  True   True
  2021-09-06  True  True   True
  2021-09-07  True  True   True
  2021-09-08  True  True   True
  
  
  
  </code></pre>