windows: support nil security descriptor on GetNamedSecurityInfo

GetNamedSecurityInfoW may return a nil security descriptor when the
object exists but has no security descriptor. This change allows
GetNamedSecurityInfo to return a nil *SECURITY_DESCRIPTOR in that case,
instead of crashing when trying to copy the nil security descriptor.

Fixes golang/go#78396

Change-Id: I2f8d26a431e0a5c3de535cf8983db1465acc24fe
Reviewed-on: https://go-review.googlesource.com/c/sys/+/760160
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>