use const& in Nullable tests

Author: Kirill Müller

inst/unitTests/cpp/misc.cpp

@@ -179,22 +179,22 @@ void test_stop_variadic() {
 }
 
 // [[Rcpp::export]]
-bool testNullableForNull(Nullable<NumericMatrix> M) {
+bool testNullableForNull(const Nullable<NumericMatrix>& M) {
     return M.isNull();
 }
 
 // [[Rcpp::export]]
-bool testNullableForNotNull(Nullable<NumericMatrix> M) {
+bool testNullableForNotNull(const Nullable<NumericMatrix>& M) {
     return M.isNotNull();
 }
 
 // [[Rcpp::export]]
-SEXP testNullableOperator(Nullable<NumericMatrix> M) {
+SEXP testNullableOperator(const Nullable<NumericMatrix>& M) {
     return M;
 }
 
 // [[Rcpp::export]]
-SEXP testNullableGet(Nullable<NumericMatrix> M) {
+SEXP testNullableGet(const Nullable<NumericMatrix>& M) {
     return M.get();
 }