Handle archive_stage=None in UploadPrebuiltsStage.
Handle archive_stage=None in UploadPrebuiltsStage.

The chromiumos-sdk builder has archive_stage=None, so UploadPrebuiltsStage
needs to handle this. This was broken by CL:46318

BUG=chromium:212327
TEST=Remote trybot runs, all unit tests.

Change-Id: I623a5f7301e58534b254e369ed53b3007f5c3294
Reviewed-on: https://gerrit.chromium.org/gerrit/46634
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Peter Mayo <3d31fdaf44dc55bb1b4f50b19df9bc879177d37a@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite