Fix Jellyfin erroring on media items without a source (#67697)

* Fix erroring on media items with a source

* code style improvement

homeassistant/components/jellyfin/media_source.py

@@ -324,6 +324,9 @@ class JellyfinSource(MediaSource):
 
 def _media_mime_type(media_item: dict[str, Any]) -> str:
     """Return the mime type of a media item."""
+    if not media_item[ITEM_KEY_MEDIA_SOURCES]:
+        raise BrowseError("Unable to determine mime type for item without media source")
+
     media_source = media_item[ITEM_KEY_MEDIA_SOURCES][0]
     path = media_source[MEDIA_SOURCE_KEY_PATH]
     mime_type, _ = mimetypes.guess_type(path)