[llvm] [Offload] Skip plugins when none requested (PR #199901)

[Weronika Lewandowska via llvm-commits]

wlemkows wrote:

This PR will no longer be needed once PR https://github.com/llvm/llvm-project/pull/202540 is merged, as the host will build correctly on Windows, and we will no longer expect the plugin list to be empty.

https://github.com/llvm/llvm-project/pull/199901