fix 导出Excel时设置合并列，指定合并列的第一行数据为null、空字符时导致后续列无法正常合并

ruoyi-common/ruoyi-common-excel/src/main/java/org/dromara/common/excel/core/CellMergeHandler.java

@@ -79,7 +79,6 @@ public class CellMergeHandler {
                     map.put(field, new RepeatCell(val, i));
                 } else {
                     RepeatCell repeatCell = map.get(field);
-                    cellValue = repeatCell.value();
                     if (isEmpty(cellValue)) {
                         // 空值跳过不合并
                         continue;