Update sdt-properties to make alias optional

src/file/table-of-contents/sdt-properties.ts

@@ -2,8 +2,10 @@
 import { StringValueElement, XmlComponent } from "@file/xml-components";
 
 export class StructuredDocumentTagProperties extends XmlComponent {
-    public constructor(alias: string) {
+    public constructor(alias?: string) {
         super("w:sdtPr");
+        if(typeof alias === 'string'){
             this.root.push(new StringValueElement("w:alias", alias));
         }
+    }
 }