In some cases in ES3, we can render without vertex attributes. For
this, don't bind or create an empty input layout, but instead bind
null.

This silences a D3D11 warning in Debug, when viewing the system out.

BUG=angleproject:667

Change-Id: Ib88dc1177d7f9bb6a5fa15e44c74da022be137f6
Reviewed-on: https://chromium-review.googlesource.com/316612Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>