title: Ag grid sorting not happening when there is a empty field
body: <p>i have a ag grid table with 10 columns, And for all the columns globally i am enabling the sorting as below,</p>
  
  <p><code>enableSorting= "true"</code><br>
  inside the <code>&lt;ag-grid-angular</code>.</p>
  
  <p>One of our column has date attribute and it contains few empty value as well. For normal list it's sorting as expected but when it comes to a list with empty value its not sorting properly.</p>
  
  <p>Is there any way i can restrict empty value to the end of the list?</p>