Work around restrictions with setting null vertex buffers on command buffers
by binding ranges of non-null buffers.

Enable VertexAttributeTest on ES2/Vulkan

BUG=angleproject:2792
BUG=angleproject:2797

Change-Id: Ief9db1e60c8c9045a4101abe859302d529decbcb
Reviewed-on: https://chromium-review.googlesource.com/1194282
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>