Vulkan: Move "null" buffer to RendererVk. This will allow the TransformFeedback and other classes to share the same buffer. Also should save a bit of memory. Bug: chromium:1086532 Change-Id: I198170b4e09165a4770b68af6df9aa7b690e8d66 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2219138 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>