Fix NullReferenceException in DidSelectFunctionalUnit on macOS

[pike00]

Fixes #831.

Problem

DeviceOperator.DidSelectFunctionalUnit dereferences functionalUnit unconditionally in its debug log line. ImageCaptureCore may invoke didSelectFunctionalUnit:error: with a null functionalUnit, so functionalUnit.GetType().Name throws NullReferenceException and scanning crashes (full trace in #831).

Change

-        _logger.LogDebug("DidSelectFunctionalUnit {Unit} {Error}", functionalUnit.GetType().Name, error);
-        SetResultOrError(_unitTcs, functionalUnit, error);
+        _logger.LogDebug("DidSelectFunctionalUnit {Unit} {Error}", functionalUnit?.GetType().Name, error);
+        SetResultOrError(_unitTcs, functionalUnit ?? _device.SelectedFunctionalUnit, error);

• Null-conditional in the log line so a null unit logs as empty instead of crashing.
• Fall back to _device.SelectedFunctionalUnit when the callback hands back a null unit, so the null does not propagate into SelectUnit/Scan.

Testing

• NAPS2.Sdk builds cleanly for net8-macos (the TFM that compiles this file).
• NAPS2.App.Mac builds and launches.
• The guard is a strict superset of prior behaviour: the ?? only activates when the value is already null, so it cannot change the existing non-null path.

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