[wikimedia] fix exception for files with empty 'metadata'

gallery_dl/extractor/wikimedia.py

@@ -77,10 +77,10 @@ class WikimediaExtractor(BaseExtractor):
 
             image["metadata"] = {
                 m["name"]: m["value"]
-                for m in image["metadata"]}
+                for m in image["metadata"] or ()}
             image["commonmetadata"] = {
                 m["name"]: m["value"]
-                for m in image["commonmetadata"]}
+                for m in image["commonmetadata"] or ()}
 
             filename = image["canonicaltitle"]
             image["filename"], _, image["extension"] = \

test/results/fandom.py

@@ -83,6 +83,15 @@ __tests__ = (
     "width"         : 728,
 },
 
+{
+    "#url"     : "https://hearthstone.fandom.com/wiki/Flame_Juggler",
+    "#comment" : "empty 'metadata'",
+    "#category": ("wikimedia", "fandom-hearthstone", "article"),
+    "#class"   : wikimedia.WikimediaArticleExtractor,
+
+    "metadata" : {},
+},
+
 {
     "#url"     : "https://projectsekai.fandom.com/wiki/Project_SEKAI_Wiki",
     "#category": ("wikimedia", "fandom-projectsekai", "article"),