MINOR MERGE REQUEST

Overview

The renter module can accept zero size files (see upload.go).

However the upload streamer will fail for an empty stream with

unable to stream an upload from a reader; upload streamer failed to get all data available

This makes a number of unit tests fail for the Sia rclone backend developed by @MSevey at https://github.com/rclone/rclone/pull/4514.

This pull request adds the appropriate check in uploadstreamer.go to make zero stream uploads work as expected.

@MSevey Please review

Edited Jun 17, 2021 by Ivan Andreev