移除添加sheet页前的null校验，write(null)时也会输出表头，而不是直接返回

src/main/java/com/alibaba/excel/write/ExcelBuilderImpl.java

@@ -50,9 +50,6 @@ public class ExcelBuilderImpl implements ExcelBuilder {
     @Override
     public void addContent(List data, WriteSheet writeSheet, WriteTable writeTable) {
         try {
-            if (data == null) {
-                return;
-            }
             context.currentSheet(writeSheet, WriteTypeEnum.ADD);
             context.currentTable(writeTable);
             if (excelWriteAddExecutor == null) {
@@ -105,9 +102,6 @@ public class ExcelBuilderImpl implements ExcelBuilder {
     @Override
     public void addContent(List data, WriteSheet writeSheet, WriteTable writeTable, String password) {
         try {
-            if (data == null) {
-                return;
-            }
             context.currentSheet(writeSheet, WriteTypeEnum.ADD);
             context.currentTable(writeTable);
             if (excelWriteAddExecutor == null) {