fix: textbook list forEach null

projects/textbook/src/app/textbook/textbook.component.ts

@@ -223,6 +223,7 @@ export class TextbookComponent implements OnInit {
   fromatFiled(list: Array<Parse.Object>, filed: string): string {
     let arr: Array<string | null> = [];
     let isDate = false;
+    // 监测空值
     list?.forEach((item: Parse.Object) => {
       // arr.add(item.get(filed))
       if (