[in_app_purchase] Avoid nullable Future.value

[simolus3]

Using Future<T>.value() with a non-nullable T is a guaranteed runtime error, which is why I'm adding an analysis warning for it.
This repo currently has one such usage in a test (it looks like the offending constructor never gets called??).

Related to flutter/flutter#84137

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[cyanglaz]

LGTM

[mvanbeusekom]

LGTM

[simolus3]

Thanks for the quick review :) Looks like the CI went through, can this get merged?