meta: review + tests (code coverage) for implicitly generated default constructors

src/entt/meta/meta.hpp

@@ -176,7 +176,7 @@ class ENTT_API meta_node {
                 nullptr,
                 nullptr,
                 0u,
-                [](typename meta_ctor_node::size_type) ENTT_NOEXCEPT -> meta_type_node * { return nullptr; },
+                nullptr,
                 [](meta_any * const) { return meta_any{std::in_place_type<Type>}; }
             };
 

test/entt/meta/meta_ctor.cpp

@@ -221,6 +221,7 @@ TEST_F(MetaCtor, ImplicitlyGeneratedDefaultConstructor) {
     // default constructor is implicitly generated
     ASSERT_EQ(counter, 1);
     ASSERT_TRUE(type.ctor<>());
+    ASSERT_EQ(type.ctor<>().arg(0), entt::meta_type{});
 
     auto any = type.ctor<>().invoke();