[mangadex] fix 'RuntimeError' for titles without a 'description' (#8389)

fixes regression introduced in 0a76cbc8c7

gallery_dl/extractor/mangadex.py

@@ -450,13 +450,13 @@ def _manga_info(self, uuid):
 
     return {
         "manga" : (mattr["title"].get("en") or
-                   next(iter(mattr["title"].values()))),
+                   next(iter(mattr["title"].values()), "")),
         "manga_id": manga["id"],
         "manga_titles": [t.popitem()[1]
                          for t in mattr.get("altTitles") or ()],
         "manga_date"  : text.parse_datetime(mattr.get("createdAt")),
         "description" : (mattr["description"].get("en") or
-                         next(iter(mattr["description"].values()))),
+                         next(iter(mattr["description"].values()), "")),
         "demographic": mattr.get("publicationDemographic"),
         "origin": mattr.get("originalLanguage"),
         "status": mattr.get("status"),

test/results/mangadex.py

@@ -178,6 +178,15 @@ __tests__ = (
     "#count"   : ">= 20",
 },
 
+{
+    "#url"     : "https://mangadex.org/title/2e73a1ac-bf26-4c04-84f6-d0a22fd37624/tomodachi-no-joshi-ni-josou-saserare-danshi",
+    "#comment" : "no 'description' (#8389)",
+    "#class"   : mangadex.MangadexMangaExtractor,
+    "#count"   : 47,
+
+    "description": "",
+},
+
 {
     "#url"     : "https://mangadex.org/titles/feed",
     "#class"   : mangadex.MangadexFeedExtractor,