exporter: silently skip unpacking unknown reference

Signed-off-by: Justin Chadwell <[email protected]>

Author: jedevc

exporter/containerimage/export.go

@@ -367,11 +367,8 @@ func (e *imageExporterInstance) unpackImage(ctx context.Context, img images.Imag
 	}
 
 	ref, ok := src.FindRef(defaultPlatform())
-	if !ok {
-		return errors.Errorf("no reference for default platform %s", defaultPlatform())
-	}
-	if ref == nil {
-		// ref has no layers, so nothing to unpack
+	if !ok || ref == nil {
+		// platform ref not found, or found platform ref has no layers, so nothing to unpack
 		return nil
 	}