Repository: flutter
Issue: [documentation ]  BackdropFilter won't apply when there is no child rendered
Labels: framework,d: api docs,P2,team-framework,triaged-framework

## Steps to Reproduce

1. Run `flutter create bug`.
2. Replace the file content with the code sample:

<details>
  <summary>Code sample</summary>

```
import 'dart:ui';
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

final url =
    "https://cdn.pixabay.com/photo/2015/04/23/22/00/tree-736885__340.jpg";

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("BackdropFilter issue"),
      ),
      body: Stack(
        children: [
          Positioned.fill(
            child: Image(
              image: NetworkImage(url),
              fit: BoxFit.cover,
            ),
          ),
          Positioned.fill(
            child: BackdropFilter(
              filter: ImageFilter.blur(sigmaX: 60, sigmaY: 60),
              child: SizedBox.expand(),
            ),
          ),
        ],
      ),
    );
  }
}
```

</details>

3. Run the app on either windows or android (any platform should be good but i tested with these two only)

## **Expected results:**
BackdropFilter would apply the blur to the background image

## **Actual results:**
Nothing happens, seems like the BackdropFilter either doesn't get the content or it doesn't render at all

<details>
  <summary>Doctor summary</summary>

```
[√] Flutter (Channel master, 1.26.0-13.0.pre.179, on Microsoft Windows [Versione 10.0.21292.1010], locale it-IT)
    • Flutter version 1.26.0-13.0.pre.179 at C:\src\flutter
    • Framework revision fc9addb88b (3 days ago), 2021-01-26 03:24:03 -0500
    • Engine revision f47ab4434d
    • Dart version 2.12.0 (build 2.12.0-257.0.dev)

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at C:\Users\dnbia\AppData\Local\Android\sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: D:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 11.0.8+10-b944.6842174)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.8.1)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.8.30711.63
    • Windows 10 SDK version 10.0.18362.0

[√] Android Studio (version 4.1.0)
    • Android Studio at D:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.8+10-b944.6842174)

[√] Connected device (3 available)
    • Mi MIX 2 (mobile) • adb-24720f95-PPnDfA._adb-tls-connect._tcp. • android-arm64  • Android 11 (API 30)
    • Windows (desktop) • windows                                    • windows-x64    • Microsoft Windows [Versione 10.0.21292.1010]
    • Chrome (web)      • chrome                                     • web-javascript • Google Chrome 88.0.4324.104

• No issues found!
```

</details>