Host an empty zip in the BCR repo

[fmeum]

This file can be used as the canonical source of modules that consist entirely of overlays and patches. An empty ZIP is very small and using the same source avoids unnecessary network connections due to almost guaranteed remote cache hits.

[fmeum]

Could we arrange for this to be hosted as e.g. https://bcr.bazel.build/empty.zip, perhaps backed by mirror.bazel.build?

[Vertexwahn]

Maybe you need to create a mirror request here -> https://github.com/bazelbuild/bazel/issues/new/choose

[fmeum]

Maybe you need to create a mirror request here -> https://github.com/bazelbuild/bazel/issues/new/choose

I do, but I would prefer a "pretty" upstream URL. Once this PR has been merged, I will send a mirror request.

[meteorcloudy]

Could we arrange for this to be hosted as e.g. https://bcr.bazel.build/empty.zip, perhaps backed by mirror.bazel.build?

Sure,https://bcr.bazel.build/empty.zip is now available! No need to mirror it again, bcr.bazel.build is already basically a mirror the BCR repo backed by a GCS bucket. It should be very stable. We could still check in this PR, but I don't think it's necessary?

[fmeum]

It's not, thanks for the prominent URL!