[camera_android_camerax] Fix setFocusMode(auto) crash when there is no auto-focus point

[motucraft]

Adds the missing isEmpty guard to the FocusMode.auto branch of AndroidCameraCameraX.setFocusMode, which read meteringPointsAf.first unconditionally and threw StateError: Bad state: No element when the current focus-and-metering action had no auto-focus point.

The FocusMode.locked branch already guards this; the auto branch did not.

Fixes flutter/flutter#187782

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Code Review

This pull request fixes a StateError in setFocusMode(FocusMode.auto) when the current focus and metering action has no auto-focus points. The implementation safely retrieves the list of auto-focus points and checks if it is empty before accessing the first element. A unit test has been added to verify this behavior, and the package version has been bumped to 0.7.2+2. There are no review comments to address.