It would help if I committed this

internal/avatar.go

@@ -0,0 +1,37 @@
+package internal
+
+import (
+	"bytes"
+	"encoding/json"
+)
+
+// sentinel value
+const deletedAvatar = "<no-avatar>"
+
+// An AvatarChange represents a change to a room's avatar. There are three cases:
+//   - an empty string represents no change, and should be omitted when JSON-serisalised;
+//   - the sentinel `avatarNotPresent` represents a room that has never had an avatar,
+//     or a room whose avatar has been removed. It is JSON-serialised as null.
+//   - All other strings represent the current avatar of the room and JSON-serialise as
+//     normal.
+type AvatarChange string
+
+func (a AvatarChange) MarshalJSON() ([]byte, error) {
+	if a == deletedAvatar {
+		return []byte(`null`), nil
+	} else {
+		return json.Marshal(string(a))
+	}
+}
+
+// Note: the unmarshalling is only used in tests.
+func (a *AvatarChange) UnmarshalJSON(data []byte) error {
+	if bytes.Equal(data, []byte("null")) {
+		*a = deletedAvatar
+		return nil
+	}
+	return json.Unmarshal(data, (*string)(a))
+}
+
+var DeletedAvatar = AvatarChange(deletedAvatar)
+var UnchangedAvatar AvatarChange