A 0-byte file produced a multipart upload with zero parts, which S3/R2 reject
with a cryptic CompleteMultipartUpload error. Guard file.size === 0 up front
with a clear message, before any S3 request is issued. Cloudflare-upload #4.
Test: tests/multipart-empty-file-guard.test.mjs.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
