EmailJSResponseStatus: improve the type

Author: Sergey Khomushin

src/models/EmailJSResponseStatus.ts

@@ -2,8 +2,8 @@ export class EmailJSResponseStatus {
   public status: number;
   public text: string;
 
-  constructor(httpResponse: XMLHttpRequest) {
-    this.status = httpResponse.status;
-    this.text = httpResponse.responseText;
+  constructor(httpResponse: XMLHttpRequest | null) {
+    this.status = httpResponse?.status || 0;
+    this.text = httpResponse?.responseText || 'network error';
   }
 }