[scrobbles] Get media_obj working again

vrobbler/apps/scrobbles/models.py

@@ -413,14 +413,7 @@ class ChartRecord(TimeStampedModel):
 
     @property
     def media_obj(self):
-        media_obj = None
-        if self.video:
-            media_obj = self.video
-        if self.track:
-            media_obj = self.track
-        if self.artist:
-            media_obj = self.artist
-        return media_obj
+        return self.item.get_concrete_model()
 
     @property
     def month_str(self) -> str: