[llvm] [SPIR-V] Fix OpGroupAsyncCopy null event substitution (PR #212754)

[Juan Manuel Martinez Caamaño via llvm-commits]

https://github.com/jmmartinez approved this pull request.

Looks good to me.

If the event is not null we still generate invalid code right? Do we have a test for that?

https://github.com/llvm/llvm-project/pull/212754