AdcDeviceCommand is now parameter-less

rewrite AdcDeviceAttribute class, basic tests of AdcDeviceAttribute passed

common/adc_traits.h

@@ -78,6 +78,15 @@ struct adc_func_traits {
     // use of an empty struct here to match std::invoke_result behaivior (at least of GCC)
 };
 
+// special case
+template <>
+struct adc_func_traits<std::nullptr_t> {
+    using ret_t = std::nullptr_t;
+    using args_t = std::tuple<>;
+    using arg1_t = std::nullptr_t;
+    static constexpr size_t arity = 0;
+};
+
 template <typename R, typename... Args>
 struct adc_func_traits<R (*)(Args...)> : adc_func_traits_helper_t<R, Args...> {
 };

common/adc_utils.h

@@ -148,6 +148,8 @@ static VT AdcTrivialDeserializer(SerializedT&& s_value)
     } else {
         static_assert(false, "TRIVIAL DESERIALIZER: UNSUPPORTED SERIALIZING TYPE!!!");
     }
+
+    return VT();
 }