meta: check on key functions rather than on context validity

src/entt/meta/meta.hpp

@@ -916,7 +916,7 @@ struct meta_data {
      * @return True if the object is valid, false otherwise.
      */
     [[nodiscard]] explicit operator bool() const noexcept {
-        return (ctx != nullptr);
+        return (node.get != nullptr);
     }
 
     /**
@@ -1045,7 +1045,7 @@ struct meta_func {
      * @return True if the object is valid, false otherwise.
      */
     [[nodiscard]] explicit operator bool() const noexcept {
-        return (ctx != nullptr);
+        return (node.invoke != nullptr);
     }
 
     /*! @copydoc meta_data::operator== */
@@ -1495,7 +1495,7 @@ public:
      * @return True if the object is valid, false otherwise.
      */
     [[nodiscard]] explicit operator bool() const noexcept {
-        return (ctx != nullptr);
+        return (node.info != nullptr);
     }
 
     /*! @copydoc meta_data::operator== */