refactor(firebase_database_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_database_platform_interface nils$ flutter analyze
Running "flutter pub get" in firebase_database_platform_interface...      1,625ms
Analyzing firebase_database_platform_interface...                       

warning • The '!' will have no effect because the receiver can't be null • test/method_channel_test.dart:32:41 • unnecessary_non_null_assertion

1 issue found. (ran in 2.7s)

Related Issues

Closes #8647
Part of #8644

Checklist

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• 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.
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Breaking Change

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• 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)

[nilsreichardt]

The e2e/ios job fails because of #8656. PR for a fix: #8657