new unit test to assess crash of print on empty factor

gtsam/linear/tests/testHessianFactor.cpp

@@ -35,6 +35,12 @@ using namespace gtsam;
 
 const double tol = 1e-5;
 
+/* ************************************************************************* */
+TEST(HessianFactor, emptyConstructor) {
+	HessianFactor f;
+//	f.print();
+}
+
 /* ************************************************************************* */
 TEST(HessianFactor, ConversionConstructor) {