[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 flutter/flutter#155210

Testing

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

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Code Review

This pull request effectively resolves a crashing bug in camera_web on Safari by safely handling cases where the facingMode capability is not supported. The fix involves using a nullable getter and null-safe operators, which is the correct approach. The addition of a new integration test to cover this specific scenario is excellent and ensures this regression won't happen again. The changelog and version updates are also correctly handled. I have one suggestion to further improve the test coverage for the related code path.