Hi, 

As define in the interface org.apache.poi.ss.usermodel.Cell, the method setStyle can accept a null value (from javadoc) : "If the value is null then the style information is removed causing the cell to used the default workbook style."

Or when I look at the implemenation in HSSFCell, I saw that: 
public void setCellStyle(CellStyle style) {
        setCellStyle( (HSSFCellStyle)style );
    }
    public void setCellStyle(HSSFCellStyle style) {
        // Verify it really does belong to our workbook
        style.verifyBelongsToWorkbook(_book);

        short styleIndex;
        if(style.getUserStyleName() != null) {
            styleIndex = applyUserCellStyle(style);
        } else {
            styleIndex = style.getIndex();
        }

        // Change our cell record to use this style
        _record.setXFIndex(styleIndex);
    }

So a null value is not checked and not taken into account.

Thanks,

Mayeul