title: Apache POI: Create a cell with type number, but to be empty
body: <p>I'm trying to create an excel document. What I need is to have a column with type number. The problem, which I have is that not every cell in this column has a value( some of them should be empty).
  I have one method, which I call in order to create the cells. I have problem  when from the methods parameters get "null", because the setCellValue(double value) can't take null as parameter. This is why I decided to check if it is null to put the cell type: empty.</p>
  
  <p><a href="https://i.stack.imgur.com/Pa74J.png" rel="nofollow noreferrer">enter image description here</a>
  <a href="https://i.stack.imgur.com/Lk4Mf.png" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>At the end what worked for me is: I just put something to the cell if the value is not null. This worked for me.</p>
  
  <pre><code>private void createCell(String cellStyle, String cellValue){
      cell = row.createCell(cellNumber++);
      cell.setCellStyle(styles.get(cellStyle));
  
      if (cellStyle.equals("numberWithDelimiters")) {
          if(cellValue!=null){
              cell.setCellValue(Integer.parseInt(cellValue));
          }
  
      }else if(cellStyle.equals("date")){
          Date date=new Date(Long.parseLong(cellValue));
          date.setHours(0);
          cell.setCellValue(date);
      }else{
      cell.setCellValue(cellValue);
      }
  }
  </code></pre>