Add some more test cases exercising result initialization from {}

test/result_value_construct2.cpp

@@ -69,6 +69,11 @@ result<std::vector<int>> fv2()
     return {{ 1, 2 }};
 }
 
+result<void> fw0()
+{
+    return {};
+}
+
 int main()
 {
     {
@@ -148,5 +153,12 @@ int main()
         BOOST_TEST_EQ( r->at(1), 2 );
     }
 
+    {
+        result<void> r = fw0();
+
+        BOOST_TEST( r.has_value() );
+        BOOST_TEST( !r.has_error() );
+    }
+
     return boost::report_errors();
 }