Vulkan: prevent volk reloading on null instance and devices Bug: angleproject:4419 Change-Id: I82f9bb157e88754efd074358dccef1bb82cab0f7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2068902 Reviewed-by: Tobin Ehlis <tobine@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>