azurestore: Fix InvalidHeaderValue error due to empty sentinel block

[DotaPie]

Need this fix for my deployment of TUSD in production. This should fix #1358.

[DotaPie]

Re-pushed commit (added GPG verification)

[Acconut]

Thank you for submitting a fix! Can you describe what you've done and how it addresses the problem?

[DotaPie]

Problem: empty sentinel block staged in NewUpload (to fix #1286) is length = 0, and azure rejects zero-length StageBlock with InvalidHeaderValue (#1358), so NewUpload fails

Downgrading to v2.9.0 was first workaround for our problem. This PR (forked from latest main) stages the sentinel as a 1-byte block under a reserved ID instead, keeping #1349's fix intact.

We built and deployed it (ghcr.io/dotapie/tusd:fix-1358) to our AKS cluster and redirecting uploads to azure blob storage now works just fine.

Edit 1: It was passing E2E test, because it is using azurite, which accepts zero-length StageBlock

[Acconut]

Thank you for fixing this issue!