ext_external_objects: Make sure tests still compile for Vulkan < 2.0
Define and cast VK_NULL_HANDLE when undefined to allow compiling against Vulkan < 2.0
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
Define and cast VK_NULL_HANDLE when undefined to allow compiling against Vulkan < 2.0