intel/vulkan: Fix clear_depth_stencil_attachment with unused attachment

When the depth_stencil_attachment is VK_ATTACHMENT_UNUSED, then
subpass->depth_stencil_attachment, so check that for NULL before
trying to dereference it.
13 jobs for !1358 with clear_unused_depth_stencil in 19 minutes and 19 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #426492
debian

00:02:30

 
  Build+Test
passed #426501
meson-arm64

00:04:51

passed #426500
meson-armhf

00:04:52

passed #426494
meson-clang

00:08:33

passed #426498
meson-clover

00:16:48

passed #426502
meson-i386

00:02:32

passed #426497
meson-main

00:05:54

passed #426493
meson-swr-glvnd

00:05:06

passed #426499
meson-vulkan

00:03:05

passed #426504
scons-llvm

00:04:10

passed #426503
scons-nollvm

00:03:51

passed #426495
scons-swr

00:05:53

passed #426496
scons-win64

00:13:48