d3d12: Handle null constant buffers

RuneScape ends up hitting this path, and it's easy enough to get
some well-defined behavior instead of a crash.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-By: Bill Kristiansen <billkris@microsoft.com>
Part-of: <!9342>
37 jobs for !9342 with d3d12-null-cbv-descriptor in 7 minutes and 57 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Sanity
passed #7670439
sanity

00:00:13

 
  Container
passed #7670445
aarch64
arm_build

00:00:16

passed #7670446
arm_test-base

00:00:16

passed #7670448
windows shell 1809 mesa
windows_build_vs2019

00:00:08

passed #7670441
x86_build-base

00:00:14

passed #7670443
x86_test-base

00:00:14

 
  Container 2
passed #7670461
android_build

00:00:19

passed #7670466
arm64_test

00:00:13

passed #7670468
armhf_test

00:00:12

passed #7670457
i386_build

00:00:18

passed #7670451
kernel+rootfs_amd64

00:00:13

passed #7670452
aarch64
kernel+rootfs_arm64

00:00:53

passed #7670453
aarch64
kernel+rootfs_armhf

00:00:10

passed #7670458
ppc64el_build

00:00:19

passed #7670460
s390x_build

00:00:18

passed #7670455
x86_build

00:00:17

passed #7670463
x86_test-gl

00:00:18

passed #7670464
x86_test-vk

00:00:14

 
  Meson X86 64
passed #7670479
meson-clang

00:01:38

passed #7670480
meson-clover

00:01:52

passed #7670473
meson-clover-testing

00:01:32

passed #7670475
gstreamer
meson-gallium

00:01:31

passed #7670477
meson-release

00:01:02

passed #7670470
meson-testing

00:02:16

passed #7670472
meson-testing-asan

00:01:55

passed #7670482
meson-vulkan

00:01:37

 
  Build Misc
passed #7670484
meson-android

00:01:01

passed #7670487
aarch64
meson-arm64

00:01:32

passed #7670489
aarch64
meson-arm64-asan

00:03:07

passed #7670490
aarch64
meson-arm64-build-test

00:01:29

passed #7670486
aarch64
meson-armhf

00:01:42

passed #7670494
meson-i386

00:01:30

passed #7670500
meson-mingw32-x86_64

00:01:24

passed #7670498
kvm
meson-ppc64el

00:01:59

passed #7670496
kvm
meson-s390x

00:03:46

passed #7670492
windows docker 1809 mesa
meson-windows-vs2019

00:04:10

 
  Layered Backends
passed #7670502
windows docker 1809 mesa
test-d3d12-windows

00:03:21