chore: 統一 可選成員 寫法

server/data/UserBean.ts

@@ -2,10 +2,10 @@ export default class UserBean {
     declare count: number
     declare id: string
     declare password: string
-    declare username: string | null
+    declare username?: string
     declare registered_time: number
     declare nickname: string
-    declare avatar_file_hash: string | null
+    declare avatar_file_hash?: string
     declare settings: string
 
     [key: string]: unknown