当QListWidget为空时如何设置居中的占位符？

from PyQt5.QtWidgets import QApplication, QListWidget, QLabel, QVBoxLayout, QWidget
from PyQt5.QtCore import Qt, QModelIndex

class PlaceholderWidget(QWidget):
    def __init__(self, text):
        super().__init__()
        self.label = QLabel(text, alignment=Qt.AlignCenter)
        layout = QVBoxLayout()
        layout.addWidget(self.label)
        self.setLayout(layout)

class CustomListWidget(QListWidget):
    def __init__(self):
        super().__init__()
        self.placeholder = PlaceholderWidget("列表为空")
        self.setIndexWidget(QModelIndex(), self.placeholder)
    
    def rowCount(self, parent=QModelIndex()):
        if self.count() == 0:
            return 1
        return super().rowCount(parent)
    
    def data(self, index, role=Qt.DisplayRole):
        if not index.isValid() and role == Qt.DisplayRole:
            return "placeholder"
        return super().data(index, role)
    
    def showPlaceholder(self):
        self.placeholder.show()
    
    def hidePlaceholder(self):
        self.placeholder.hide()

# 创建应用程序和窗口
app = QApplication([])
window = CustomListWidget()
window.show()

# 添加数据到列表
window.addItem("Item 1")
window.addItem("Item 2")

# 当列表为空时显示占位符
if window.count() == 0:
    window.showPlaceholder()
else:
    window.hidePlaceholder()

# 运行应用程序
app.exec_()