Why not allowing to parse null as std::optional in yaml files via `is_empty(null) -> true`

[Fellfalla]

Hey,

i was interested in explicitly point out "null" for std::nullopt values inside yaml files.
Would there a specific reason against applying this patch

--- a/include/rfl/yaml/Reader.hpp
+++ b/include/rfl/yaml/Reader.hpp
@@ -68,7 +68,7 @@
   }

   bool is_empty(const InputVarType& _var) const noexcept {
-    return !_var.node_ && true;
+    return !_var.node_ || _var.node_.IsNull();
   }

   template <class T>

[liuzicheng1987]

@Fellfalla I don't see anything wrong with it. PR is welcome.

[Fellfalla]

@liuzicheng1987 #654