fix: videoDetail cover null

lib/common/pages_bottom_sheet.dart

@@ -45,7 +45,7 @@ class EpisodeBottomSheet {
         title = '第${episode.title}话  ${episode.longTitle!}';
         break;
     }
-    return isFullScreen || episode?.cover == null
+    return isFullScreen || episode?.cover == null || episode?.cover == ''
         ? ListTile(
             onTap: () {
               SmartDialog.showToast('切换至「$title」');

lib/models/video_detail_res.dart

@@ -412,8 +412,8 @@ class Part {
     dimension = json["dimension"] == null
         ? null
         : Dimension.fromJson(json["dimension"]);
-    firstFrame = json["first_frame"];
-    cover = json["first_frame"];
+    firstFrame = json["first_frame"] ?? '';
+    cover = json["first_frame"] ?? '';
   }
 
   Map<String, dynamic> toJson() {