fix(battery_plus): Fix return value of getBattery to be nullable

[koji-1009]

Description

Fixed an exception that occurred when calling getBattery() in firefox and safari.
In cases where getBattery() is undefined, you need to declare JSPromise<BatteryManager>? instead of JSPromise<BatteryManager>.

The example code has been reverted to the previous one, as the try-catch is now correct.

Related Issues

• Related feat(battery_plus)!: Migrate to package:web #2720

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[koji-1009]

demo: https://koji-1009.github.io/plus_plugins/
demo pr: koji-1009#7

[miquelbeltran]

Thank you!