refactor: make CameraFacing non null

lib/src/qr_code_scanner.dart

@@ -156,14 +156,14 @@ class _QRViewState extends State<QRView> {
 
 class _QrCameraSettings {
   _QrCameraSettings({
-    this.cameraFacing,
+    this.cameraFacing = CameraFacing.unknown,
   });
 
-  final CameraFacing? cameraFacing;
+  final CameraFacing cameraFacing;
 
   Map<String, dynamic> toMap() {
     return <String, dynamic>{
-      'cameraFacing': cameraFacing!.index,
+      'cameraFacing': cameraFacing.index,
     };
   }
 }

lib/src/types/camera.dart

@@ -3,5 +3,8 @@ enum CameraFacing {
   back,
 
   /// Shows front facing camera.
-  front
+  front,
+
+  /// Unknown camera
+  unknown
 }