title: GAS : How to format all empty cells in a sheet
body: <p>I'd like to format all empty/blank cells in a whole sheet with a white background but I don't find the best method to retrieve a range that can handle all those empty/blank cells.
  I don't want to create a conditional rule, I'd like to handle it within a script.</p>
  <p>What kind of method should I use ?
  Thanks a lot !</p>
  <p>P.S : I'm a newbie :)</p>