Prevent empty chapter updates on migration

In case there were no chapters to migrate the read or bookmark status, a request with an empty list was sent to the server.
This caused an exception on the server which resulted in a request failure which at the end caused the migration to fail

src/lib/data/Mangas.ts

@@ -378,8 +378,9 @@ export class Mangas {
         });
 
         await Promise.all([
-            requestManager.updateChapters(readChapters, { isRead: true }).response,
+            readChapters.length && requestManager.updateChapters(readChapters, { isRead: true }).response,
-            requestManager.updateChapters(bookmarkedChapters, { isBookmarked: true }).response,
+            bookmarkedChapters.length &&
+                requestManager.updateChapters(bookmarkedChapters, { isBookmarked: true }).response,
         ]);
     }