如何在TextFormField中显示最初只允许flutter中的数字的空字符串？

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Number Input'),
        ),
        body: Center(
          child: Padding(
            padding: EdgeInsets.all(16.0),
            child: MyForm(),
          ),
        ),
      ),
    );
  }
}

class MyForm extends StatefulWidget {
  @override
  _MyFormState createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  TextEditingController _controller;

  @override
  void initState() {
    super.initState();
    _controller = TextEditingController();
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return TextFormField(
      controller: _controller,
      inputFormatters: [
        WhitelistingTextInputFormatter.digitsOnly,
      ],
      decoration: InputDecoration(
        labelText: 'Number',
      ),
    );
  }
}