title: date conditional formatting in xlsxwriter formatting blank cells and cells that meet criteria
body: <p>I have a date column I am trying to format with xlsxwriter. </p>
  
  <p>The column can have a date in it or be blank. </p>
  
  <p>The conditional formatting (highlight cell when date is &lt; today) I am trying to apply is working. But it is also applying the same formatting rules to the blank cells. </p>
  
  <p>How can I prevent it from formatting the blank cells? </p>
  
  <p>See below for code:</p>
  
  <pre><code>
  format1 = workbook.add_format({'bg_color':   '#FFC7CE',
                                 'font_color': '#9C0006'})
  date = datetime.now()
  
  worksheet.conditional_format(due_range, {'type':     'date',
                                          'criteria': 'less than',
                                          'value':    date,
                                          'format':   format1})
  
  writer.save()
  workbook.close()
  
  </code></pre>