Fix crash admin media list api when info is None (#14537)

Fixes matrix-org/synapse#14536

Author: schmop

changelog.d/14537.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where the [List media admin API](https://matrix-org.github.io/synapse/latest/admin_api/media_admin_api.html#list-all-media-in-a-room) would fail when processing an image with broken thumbnail information.

synapse/storage/databases/main/room.py

@@ -912,7 +912,11 @@ def _get_media_mxcs_in_room_txn(
                 event_json = db_to_json(content_json)
                 content = event_json["content"]
                 content_url = content.get("url")
-                thumbnail_url = content.get("info", {}).get("thumbnail_url")
+                info = content.get("info")
+                if isinstance(info, dict):
+                    thumbnail_url = info.get("thumbnail_url")
+                else:
+                    thumbnail_url = None
 
                 for url in (content_url, thumbnail_url):
                     if not url: