Fix test that passes a nil mutator to 'move'

The call is incorrect, but the test does not fail, because it enables
dry-run, and that does not exercise the mutators.

cmd/clusterctl/client/cluster/mover_test.go

@@ -1203,7 +1203,7 @@ func Test_objectMover_move_dryRun(t *testing.T) {
 				dryRun:    true,
 			}
 
-			err := mover.move(ctx, graph, toProxy, nil)
+			err := mover.move(ctx, graph, toProxy)
 			if tt.wantErr {
 				g.Expect(err).To(HaveOccurred())
 				return