UniqueHandle- Fixed compile error in 32-bit

Because in 32-bit Vulkan-handles are integers, the assignment from
nullptr is not valid and doesn't compile. VK_NULL_HANDLE on the other
hand abstracts the differences between 32- and 64-bit environments.

VulkanHppGenerator.cpp

@@ -515,7 +515,7 @@ std::string const uniqueHandleHeader = { R"(
     Type release()
     {
       Type value = m_value;
-      m_value = nullptr;
+      m_value = VK_NULL_HANDLE;
       return value;
     }
 

vulkan/vulkan.hpp

@@ -397,7 +397,7 @@ namespace vk
     Type release()
     {
       Type value = m_value;
-      m_value = nullptr;
+      m_value = VK_NULL_HANDLE;
       return value;
     }