fix: readAllNotifications message not working (#14374)

* refactor: add and use isJsonObject

* fix: readNotification message without body is not working

* docs(changelog): WSの`readAllNotifications` メッセージが `body` を持たない場合に動作しない問題

* Update CHANGELOG.md

Co-authored-by: Sayamame-beans <61457993+Sayamame-beans@users.noreply.github.com>

---------

Co-authored-by: Sayamame-beans <61457993+Sayamame-beans@users.noreply.github.com>

packages/backend/src/misc/json-value.ts

@@ -6,3 +6,7 @@
 export type JsonValue = JsonArray | JsonObject | string | number | boolean | null;
 export type JsonObject = {[K in string]?: JsonValue};
 export type JsonArray = JsonValue[];
+
+export function isJsonObject(value: JsonValue | undefined): value is JsonObject {
+	return typeof value === 'object' && value !== null && !Array.isArray(value);
+}