[camera_web] Fixed crashing bug on camera_web 0.3.5 for Safari (#10595)

On certain browsers (Safari), getting `facingMode` on video track capabilities is not supported. The code first checks if `facingMode` is available on `MediaTrackSettings` using `facingModeNullable` but when the code checks for facingMode on `MediaTrackCapabilities`, it uses the non nullable `facingMode` getter instead of `facingModeNullable`. This causes a crash and the camera isn't initialized correctly.

This is a blocker for reliably deploying camera_web 0.3.5 which enables people to use WASM with camera functionality.

Fixes https://github.com/flutter/flutter/issues/155210

## Testing

Tested on Safari on Mac and iPhone with camera initialization - camera now successfully initializes where it previously failed.

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