Serialize empty tuple into '[]' instead of null (#4594)

Signed-off-by: Michael Valladolid <mikevalladolid@gmail.com>

include/nlohmann/detail/conversions/from_json.hpp

@@ -448,6 +448,12 @@ std::tuple<Args...> from_json_tuple_impl_base(BasicJsonType&& j, index_sequence<
     return std::make_tuple(std::forward<BasicJsonType>(j).at(Idx).template get<Args>()...);
 }
 
+template<typename BasicJsonType>
+std::tuple<> from_json_tuple_impl_base(BasicJsonType& /*unused*/, index_sequence<> /*unused*/)
+{
+    return {};
+}
+
 template < typename BasicJsonType, class A1, class A2 >
 std::pair<A1, A2> from_json_tuple_impl(BasicJsonType&& j, identity_tag<std::pair<A1, A2>> /*unused*/, priority_tag<0> /*unused*/)
 {