Some D3D11 drivers may produce unexpected results when blending is
apllied to integer attachments.

Minor cleanup: early exit for nullptr attachments.

New BlendIntegerTest in angle_end2end.

Bug: angleproject:4548, angleproject:4547, angleproject:4525, angleproject:4394
Change-Id: I236ac6db1f8dcaca54e9bc1b0803e1775fd75632
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2134448
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>