[llvm] [Offload] Allow setting null arguments in olLaunchKernel and fix amdgpu handling for empty kernel arguments (PR #141958)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 Thanks for looking into this. I've rebased on top of your change and, with another small amdgpu modification, it looks like the test is passing.

Specifically, if we don't specify `KernelArgs`, we now skip the schedReleaseBuffer callback action entirely.

https://github.com/llvm/llvm-project/pull/141958