[camera] Make Optional.of constructor const (#11247) Resolves old TODO from 2023 ( https://github.com/flutter/packages/commit/6e09f5b0d26f77bf70ab27d6ab4c7f8b46968c34 committed by @camsim99 ) by making the `Optional.of` constructor `const` and removing the obsolete runtime `ArgumentError.checkNotNull` guard that was only needed during Dart 2's mixed-mode (unsound null safety) era. Mixed-mode execution was removed in Dart 3.0, so this is dead code. Not a breaking change. Note that the entire `Optional<T>` class (and surrounding `CameraController`) is copy-pasted across 4 files in the camera packages. Would you prefer to move `Optional<T>` into `camera_platform_interface` to share it, or replace it entirely e.g. with a private sentinel pattern in `copyWith` (as the Flutter framework does for `ThemeData`)? ## Pre-Review Checklist
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