fix:WebpageShareModel

lib/src/models/wechat_share_models.dart

@@ -273,15 +273,13 @@ class WeChatShareWebPageModel extends WeChatShareModel {
     @required this.webPage,
     this.title: "",
     this.description: "",
-    String thumbnail,
+    this.thumbnail,
     WeChatScene scene,
     String messageExt,
     String messageAction,
     String mediaTagName,
   })  : this.transaction = transaction ?? "text",
-        this.thumbnail = thumbnail ?? "",
         assert(webPage != null),
-        assert(thumbnail != null),
         super(
             mediaTagName: mediaTagName,
             messageAction: messageAction,