turnip: Use VK_NULL_HANDLE instead of NULL.

Only occurrence of implicitly converting pointer->int.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2570>

src/freedreno/vulkan/tu_pipeline.c

@@ -1980,7 +1980,7 @@ tu_graphics_pipeline_create(VkDevice device,
    if (result == VK_SUCCESS)
       *pPipeline = tu_pipeline_to_handle(pipeline);
    else
-      *pPipeline = NULL;
+      *pPipeline = VK_NULL_HANDLE;
 
    return result;
 }