The SPIR-V and HLSL sanitizing transforms add an empty one if
necessary.
Fixed: tint:679
Change-Id: Ic98ff3109d7381b1fbc2de68d95d57e15c7a67c0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/46700
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>