[web] Ensure handleNavigationMessage can receive null arguments.

[ditman]

In a recent update to handleNavigationMessage some extra messages were added that don't require any arguments. From the framework side, those messages are sent without arguments (OK), but the engine was never modified to accept null arguments, so it fails with:

Error: Expected a value of type 'Map<String, dynamic>', but got one of type 'Null'
...
    at _engine.EngineSingletonFlutterWindow.new.handleNavigationMessage 
...
    at _engine.EnginePlatformDispatcher.__.sendPlatformMessage 
...
    at OptionalMethodChannel.invokeMethod
...
    at Function.selectSingleEntryHistory (framework)

Since that unhandled exception bubbles up to the browser, web apps in master display a red error message, which may break all web integration tests like so:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞═════════════════
The following TypeErrorImpl was thrown running a test:
Expected a value of type 'Map<String, dynamic>', but got one of type 'Null'

When the exception was thrown, this was the stack:
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 251:49      throw_
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 84:3        castError
...
lib/_engine/engine/window.dart 116:32                                             handleNavigationMessage
...
packages/flutter/src/services/system_channels.dart.js 1007:20                     invokeMethod
packages/flutter/src/services/system_navigator.dart.js 26:56                      selectSingleEntryHistory
packages/flutter/src/widgets/inherited_model.dart.js 25905:42                     initState

Fixes flutter/flutter#83158

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.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.
• The reviewer has submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

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