feat: migrate macos/macos_device.dart to null safety

[wanglikun7342]

migrate macos/macos_device.dart to null safety

List which issues are fixed by this PR. You must list at least one issue.
#71511

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.
NO

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[wanglikun7342]

@jmagman null safe mirgate pr

[wanglikun7342]

@Hixie this pr need an test exemption

[jmagman]

@Hixie this pr need an test exemption

Instead, please try to migrate the corresponding test, migrating tests at the same time can catch crashes. https://github.com/flutter/flutter/blob/master/packages/flutter_tools/test/general.shard/macos/macos_device_test.dart

[jmagman]

LGTM

[christopherfujino]

LGTM