[macos] early return to suppress clang-tidy warning (flutter/engine#56588)
If we disable the NSAssert during release mode, this code will continue after NSAssert failure, which will then pass a `nil` value into a dictionary (which cannot have a nil value), hence the clang-tidy warning here https://github.com/flutter/engine/pull/53005#issuecomment-2140975126
*List which issues are fixed by this PR. You must list at least one issue.*
https://github.com/flutter/flutter/issues/148279
https://github.com/flutter/flutter/issues/157837
*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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