Handle empty thumbnail_url when refreshing covers

app/src/main/java/eu/kanade/tachiyomi/data/library/LibraryUpdateService.kt

@@ -337,8 +337,8 @@ class LibraryUpdateService(
         if (preferences.autoUpdateMetadata()) {
             source.fetchMangaDetails(manga)
                 .map { updatedManga ->
-                    // Avoid "losing" covers
-                    if (updatedManga.thumbnail_url != null) {
+                    // Avoid "losing" existing cover
+                    if (!updatedManga.thumbnail_url.isNullOrEmpty()) {
                         manga.prepUpdateCover(coverCache, updatedManga, false)
                     } else {
                         updatedManga.thumbnail_url = manga.thumbnail_url

app/src/main/java/eu/kanade/tachiyomi/util/MangaExtensions.kt

@@ -17,6 +17,9 @@ fun Manga.prepUpdateCover(coverCache: CoverCache, remoteManga: SManga, refreshSa
     // Never refresh covers if the new url is null, as the current url has possibly become invalid
     val newUrl = remoteManga.thumbnail_url ?: return
 
+    // Never refresh covers if the url is empty to avoid "losing" existing covers
+    if (newUrl.isEmpty()) return
+
     if (!refreshSameUrl && thumbnail_url == newUrl) return
 
     when {