title: Ensure Cell is NOT BLANK
body: <p>By observation I believe that data validation is only triggered when some data is entered into a cell. Is this correct?</p>
  
  <p>Is it possible to trigger a validation such as <code>NOT(ISBLANK(...))</code> if the cell is blank? eg: the data validation warning message.</p>
  
  <p>What I want to do is give a validation error IF the cell IS BLANK.</p>
  
  <p>Is this possible?</p>