Test data not being uploaded for xdist enabled but numprocesses set to 0

[zpoint]

In SkyPilot, we disabled xdist parallelism on the Buildkite CI environment but allow it locally and in other environments to reduce memory usage on Buildkite.(See #7794)

So we need xdist enabled but override the numprocesses option on the fly.

This results in the xdist environment being set, and numprocesses also being set but potentially set to 0 in special cases.

In this case, the plugin doesn't upload any data, So we collect no data in our buildkite env.

This PR fixes the issue by treating this case as if xdist is not enabled, allowing the main thread to upload the data.