Add nullable check

[HeyImChris]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

There's one more spot we need a nullable type specifier due to downstream deps having the -Wnullability-completeness flag enabled.

Test Plan

Shouldn't affect user scenarios, but if it builds then should be all set.

[pull-bot]

	Warnings
⚠️	❔ Base Branch - The base branch for this PR is something other than master. Are you sure you want to merge these changes into a stable release? If you are interested in backporting updates to an older release, the suggested approach is to land those changes on master first and then cherry-pick the commits into the branch for that release. The Releases Guide has more information.

Messages

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📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General macOS iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.

Generated by 🚫 dangerJS against 8bfd961

[harrieshin]

similar to other pr, are we suppressing some warning?

[HeyImChris]

similar to other pr, are we suppressing some warning?

These warnings aren't suppressed but downstream we have warnings set to throw errors which breaks our builds. We still get the warnings here but no one's taken the time to fix these warnings yet.

[anandrajeswaran]

similar to other pr, are we suppressing some warning?

These warnings aren't suppressed but downstream we have warnings set to throw errors which breaks our builds. We still get the warnings here but no one's taken the time to fix these warnings yet.

I would argue that a warning not being treated as an error means it is effectively suppressed. If it doesn't break CI there isn't any real practical difference between whether a warning shows up or it doesn't. Ultimately should try to turn on warnings as errors and push that upstream as soon as possible as it really shouldn't be too bad to eliminate this class of failures if indeed we have it solved downstream.