Add to_json for additional safety

roles/custom/matrix-bridge-mautrix-signal/templates/config.yaml.j2

@@ -112,7 +112,7 @@ bridge:
     # Should the Signal user's phone number be included in the room topic in private chat portal rooms?
     number_in_topic: true
     # Avatar image for the Note to Self room.
-    note_to_self_avatar: {{ matrix_mautrix_signal_bridge_note_to_self_avatar }}
+    note_to_self_avatar: {{ matrix_mautrix_signal_bridge_note_to_self_avatar | to_json }}
 
     portal_message_buffer: 128