[FormBuilderSignaturePad]: isDirty always true

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Package/Plugin version

9.1.1

### Platforms

- [X] Android
- [ ] iOS
- [ ] Linux
- [ ] MacOS
- [ ] Web
- [ ] Windows

### Flutter doctor

Flutter doctor

```bash
[✓] Flutter (Channel stable, 3.16.0, on macOS 14.1.1 23B81 darwin-arm64, locale es-ES)
• Flutter version 3.16.0 on channel stable at /Users/miguel/bin/flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision db7ef5bf9f (13 days ago), 2023-11-15 11:25:44 -0800
• Engine revision 74d16627b9
• Dart version 3.2.0
• DevTools version 2.28.2

[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
• Android SDK at /Users/miguel/Library/Android/sdk
• Platform android-34, build-tools 33.0.1
• Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 17.0.6+0-17.0.6b829.9-10027231)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.0.1)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Build 15A507
• CocoaPods version 1.13.0

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2022.3)
• Android Studio at /Applications/Android Studio.app/Contents
• 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 17.0.6+0-17.0.6b829.9-10027231)

[✓] VS Code (version 1.84.2)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.56.0

[✓] Connected device (4 available)
• SM A125F (mobile) • R58R54D6Y3K • android-arm64 • Android 12 (API 31)
• iPhone de Miguel Angel (mobile) • 00008110-000414892152401E • ios • iOS 17.1.1 21B91
• macOS (desktop) • macos • darwin-arm64 • macOS 14.1.1 23B81 darwin-arm64
• Chrome (web) • chrome • web-javascript • Google Chrome 119.0.6045.159

[✓] Network resources
• All expected network resources are available.

• No issues found!
```

### Minimal code example

Code sample

```dart
FormBuilderSignaturePad(
name: 'signature',
onSaved: (v) {
print(v);
},
),
```

### Current Behavior

Field is always in dirty state

### Expected Behavior

Dirty should be only true when fields value has changed

### Steps To Reproduce

1. Copy the example field to a working form
2. Submit form and check that 'signature' field is dirty although you don't touch anything.

### Aditional information

_No response_