S3Store: Deferred length uploads fail when only 1 incomplete part is uploaded

[atticoos]

When uploading a readable stream (deferred length), when the overall upload is smaller than the minPartSize, then the upload remains as an incomplete part, never establishing an actual part for the S3 multipart upload to complete.

Reproducible Steps

1. Upload via readable stream with deferred lengths
2. Upload a very small set of chunks that are collectively smaller than the minPartSize

Expected behavior

The file upload completes

Actual behavior

500 - finishMultipartUpload fails because there are no completed parts resolved from client.listParts.

As a result, S3 contains no Parts in its response field, resulting in an undefined error when finishMultipartUpload attempts parts.map

Fix - #502