Introduce constructors on nullptr_t for the RAII handle classes.

vulkan/vulkan.hpp

@@ -1385,15 +1385,24 @@ namespace VULKAN_HPP_NAMESPACE
 
   class DispatchLoaderBase
   {
-#if !defined( NDEBUG )
   public:
+    DispatchLoaderBase() = default;
+    DispatchLoaderBase( std::nullptr_t )
+#if !defined( NDEBUG )
+      : m_valid( false )
+#endif
+    {}
+
+#if !defined( NDEBUG )
     size_t getVkHeaderVersion() const
     {
+      VULKAN_HPP_ASSERT( m_valid );
       return vkHeaderVersion;
     }
 
   private:
     size_t vkHeaderVersion = VK_HEADER_VERSION;
+    bool   m_valid         = true;
 #endif
   };