getCookies throws a RangeError if there are no cookies

## System info

Issue occurs on: Android for sure, haven't tested iOS
Plugin version: 0.3.7
Flutter doctor output:

```
[√] Flutter (Channel stable, v1.7.8+hotfix.4, on Microsoft Windows [Version 10.0.17763.678], locale de-AT)

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.1)
[√] Android Studio (version 3.4)
[√] Connected device (3 available)

• No issues found!
```

## Steps to Reproduce

1. Call `FlutterWebviewPlugin.getCookies` when there are no cookies, e.g. directly after calling `FlutterWebviewPlugin.cleanCookies`.
2. Note: Debugging shows that the variable `cookiesString` in the `getCookies` method has the value `\"\"` when the error is thrown.

## Logs

```
E/flutter: [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: RangeError (index): Invalid value: Only valid value is 0: 1
#0 List.[] (dart:core-patch/growable_array.dart:145:60)
#1 FlutterWebviewPlugin.getCookies. (package:flutter_webview_plugin/src/base.dart:238:34)
#2 List.forEach (dart:core-patch/growable_array.dart:278:8)
#3 FlutterWebviewPlugin.getCookies (package:flutter_webview_plugin/src/base.dart:236:32)

#4 _SettingsScreenState._getCookieCount (package:my_app/screens/settings_screen.dart:37:43)
[rest is not relevant]
```

```
No issues found! (ran in 2.7s)
```

```
[√] Flutter (Channel stable, v1.7.8+hotfix.4, on Microsoft Windows [Version 10.0.17763.678], locale de-AT)
• Flutter version 1.7.8+hotfix.4 at C:\Android\flutter
• Framework revision 20e59316b8 (4 weeks ago), 2019-07-18 20:04:33 -0700
• Engine revision fee001c93f
• Dart version 2.4.0

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.1)
• Android SDK at C:\Android\Sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.1
• ANDROID_HOME = C:\Android\Sdk
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)
• All Android licenses accepted.

[√] Android Studio (version 3.4)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin version 38.2.1
• Dart plugin version 183.6270
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[√] Connected device (1 available)
• Android SDK built for x86 • emulator-5554 • android-x86 • Android 9 (API 28) (emulator)

• No issues found!
```

Commencez dans package:flutter_webview_plugin/src/base.dart, autour de FlutterWebviewPlugin.getCookies aux lignes 236-238, et reproduisez le cas après avoir appelé cleanCookies. Vérifiez qu’une réponse de cookies vide ne provoque plus de RangeError et que getCookies gère correctement le cas sans cookies.

Rédigé par le modèle d'indexation à partir du texte de l'issue.