如果TextFormField中没有输入值，如何将字段作为'null‘保存到Firebase

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

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

class _MyFormState extends State<MyForm> {
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  String _textFieldValue;

  void _submitForm() {
    if (_formKey.currentState.validate()) {
      _formKey.currentState.save();

      // Check if the text field value is empty
      var fieldValue = _textFieldValue.isEmpty ? null : _textFieldValue;

      // Save the field value to Firebase Firestore
      FirebaseFirestore.instance
          .collection('myCollection')
          .doc('myDocument')
          .set({'myField': fieldValue})
          .then((value) {
        // Data saved successfully
        print('Data saved to Firestore');
      }).catchError((error) {
        // Error occurred while saving data
        print('Error: $error');
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('My Form'),
      ),
      body: Form(
        key: _formKey,
        child: Column(
          children: [
            TextFormField(
              decoration: InputDecoration(labelText: 'My Field'),
              onSaved: (value) {
                _textFieldValue = value;
              },
            ),
            RaisedButton(
              child: Text('Submit'),
              onPressed: _submitForm,
            ),
          ],
        ),
      ),
    );
  }
}