Fix "You must specify at least one part" error

[vlasisPit]

After error `BadDigest Message: The CRC64NVME you specified did not match the calculated checksum.` uploading a part fails. Then error `You must specify at least one part` appears because the CompleteUpload can not succeed

This PR adds checks to ensure completed_parts is not empty before calling CompleteMultipartUpload

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TYPE: IMPROVEMENT
DESC: Fix "You must specify at least one part" error

[vlasisPit]

Is this one still needed, or should we close it?

[teo-tsirpanis]

I think it would be better to PutObject an empty object if the multipart upload ends up having no parts.

Also, if uploading a part fails, should we instead remove it from the states table?

[ypatia]

Also, if uploading a part fails, should we instead remove it from the states table?

@teo-tsirpanis I created https://linear.app/tiledb/issue/CORE-503/add-proper-cleanup-after-an-error-occurs-in-multi-part-uploads for following up on proper multi-part upload handling.

I think it would be better to PutObject an empty object if the multipart upload ends up having no parts.

Could you push a fix about your suggestion? Otherwise we can close this PR and fix it as part of CORE-503

[teo-tsirpanis]

On second thought, this change looks good for now. The completed parts won't be empty unless uploading a part has failed, in which case we should not upload an empty object.