refactor(firebase_dynamic_links_platform_interface): fix analyzer issue introduced in Flutter 3.0.0

[nilsreichardt]

Description

Flutter 3.0.0 caused failing the CI: https://github.com/firebase/flutterfire/runs/6398175458?check_suite_focus=true

Nils-MBP-13-M1-3:firebase_dynamic_links_platform_interface nils$ flutter analyze
Running "flutter pub get" in firebase_dynamic_links_platform_interface...      1,220ms
Analyzing firebase_dynamic_links_platform_interface...                  

warning • The '!' will have no effect because the receiver can't be null • test/mock.dart:78:33 • unnecessary_non_null_assertion

1 issue found. (ran in 2.5s)

Related Issues

Closes #8646
Part of #8644

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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

• Yes, this is a breaking change.
• No, this is not a breaking change.

[nilsreichardt]

cc: @Salakar (tagging you because it fixes the broken CI pipeline, see #8644)