This updates AccessibilityBridge::AddFlutterSemanticsNodeUpdate and AccessibilityBridge::AddFlutterSemanticsCustomActionUpdate to accept references instead of pointers as null isn't an acceptable input and these aren't out parameters.

This code is shared by both the macOS and Windows embedders but not the Linux embedder.

Part of: flutter/flutter#121176

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.

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