feat(connectivity): improve network callback unregistration handling

[KirillBorodin]

PR Description

fix: safely unregister Android network callback to prevent crash when callback already unregistered or null

Problem

The app crashes with IllegalArgumentException: NetworkCallback was not registered when the Flutter engine is destroyed, particularly during background work with WorkManager. This happens because Android's ConnectivityManager checks if networkCallback.networkRequest is null before unregistering:

if (networkCallback.networkRequest == null) {
    throw new IllegalArgumentException("NetworkCallback was not registered");
}

Since networkRequest is an internal field that we cannot check directly, and it can become null in various scenarios (double cleanup, process recreation, etc.), we need to handle the unregistration defensively.

Related Issues

#1025

Checklist

- [x] I read the [Contributor Guide](https://github.com/fluttercommunity/plus_plugins/blob/main/CONTRIBUTING.md) and followed the process outlined there for submitting PRs.
- [x] I titled the PR using Conventional Commits.
- [x] I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
- [x] All existing and new tests are passing.
- [x] The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does this PR require plugin users to manually update their apps?

- [ ] Yes, this is a breaking change (add ! in the title).
- [x] No, this is *not* a breaking change.