fix: apply excel format for blank cells

Fixes #159

Author: mlopezFC

src/main/java/com/flowingcode/vaadin/addons/gridexporter/ExcelStreamResourceWriter.java

@@ -357,6 +357,9 @@ private void buildCell(Object value, Cell cell, Column<T> column, T item) {
     }
     if (value == null) {
       PoiHelper.setBlank(cell);
+      if(excelFormat != null) {       
+        applyExcelFormat(cell, excelFormat, cellStyles);
+      }
     } else if (value instanceof Number) {
       excelFormat = (excelFormat!=null)?excelFormat:"0";
       cell.setCellValue(((Number) value).doubleValue());