When we have xfb descriptor set without buffer bindings it causes
errors in Vulkan validation layer. So we set empty buffer by default.

BUG=angleproject:3681
Tests: dEQP-GLES3.functional.*transform_feedback*
dEQP-GLES3.functional.lifetime.attach.deleted_input.buffer_vertex_array

Change-Id: I4e7c9c9ee661f1fc92bf01fb78a2212c1a423eaa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1753387
Commit-Queue: Hyunchang Kim <hckim.kim@samsung.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>