fix(BillReminders): 修复金额显示的空值判断

- 修改金额判断条件，兼容 null 和 undefined 值
- 优化渲染逻辑，避免金额为 null 时错误渲染
- 保持界面金额显示样式一致

components/BillReminders.tsx

@@ -204,7 +204,7 @@ const BillReminders: React.FC<BillRemindersProps> = ({ reminders, onAdd, onUpdat
                                     {r.isCompleted && <svg className="w-3 h-3 text-white" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={3} d="M5 13l4 4L19 7" /></svg>}
                                 </button>
                                 <span className={`font-bold text-sm truncate ${r.isCompleted ? 'line-through text-gray-400' : 'text-gray-800'}`}>{r.title}</span>
-                                {r.amount !== undefined && (
+                                {r.amount != null && (
                                    <span className="font-mono text-gray-500 font-bold text-[10px] bg-gray-50 px-1 rounded">¥{r.amount}</span>
                                 )}
                            </div>