[camera_web] Fixed crashing bug on camera_web 0.3.5 for Safari

[jalen-videotape]

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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