Vulkan: Use VK_NULL_HANDLE for handles, fixes 32-bit builds

Some Vulkan types are defined as "non dispatchable handles" and use a
different typedef on 32-bit and 64-bit systems (struct pointer on
64-bit, `uint64_t` otherwise).

https://github.com/KhronosGroup/Vulkan-Headers/blob/0e78ffd1dcfc3e9f14a966b9660dbc59bd967c5c/include/vulkan/vulkan_core.h#L59-L65

Contrarily to `NULL`, `nullptr` can't be converted to `uint64_t` so
build was now failing on 32-bit after converting the codebase from
using `NULL` to `nullptr`.

Fixes #37620.

drivers/vulkan/vulkan_context.cpp

@@ -937,7 +937,7 @@ Error VulkanContext::_update_swap_chain(Window *window) {
 		/*compositeAlpha*/ compositeAlpha,
 		/*presentMode*/ swapchainPresentMode,
 		/*clipped*/ true,
-		/*oldSwapchain*/ nullptr,
+		/*oldSwapchain*/ VK_NULL_HANDLE,
 	};
 
 	err = fpCreateSwapchainKHR(device, &swapchain_ci, nullptr, &window->swapchain);