@tus/s3-store: gracefully handle zero byte uploads (#824)

This PR resolves an issue with zero byte uploads on S3 or Minio.

## Error message

- S3: `MalformedXML: The XML you provided was not well-formed or did not
validate against our published schema`
- Minio: `InvalidRequest: You must specify at least one part`

## Reproduction

Any upload with zero bytes will reproduce this issue. This is a simple
repro using `tus-js-client`

```JavaScript
import * as tus from 'tus-js-client'
const upload = new tus.Upload(Readable.from(Buffer.alloc(0)), {
  endpoint,
  chunkSize: 1024,
  uploadSize: 0,
  retryDelays: [],
  uploadDataDuringCreation: true,
  metadata: {
    bucketName: 'my-bucket',
    objectName: 'some/object/path.ext',
  },
  onError: (error) => {
    console.error("onError", error)
  },
  onSuccess: () => {
    console.log(`onSuccess`)
  },
})

upload.start()
```

## Additional context

Including the `StreamLimiter` transform as is done [here in
BaseHandler](https://github.com/tus/tus-node-server/blob/8906f1454f171a4b964bcf7a4d6fa41a0636f2b9/packages/server/src/handlers/BaseHandler.ts#L189)
causes zero byte chunks to not be emitted. This means that
`chunkFinished` is never called and therefore no part is ever created.
This results in the error as described above because you cannot complete
a multipart upload that has no parts.

The test suite already has a test for zero byte uploads, but it doesn't
use `StreamLimiter` so it succeeds. I've added a mostly identical test
that includes `StreamLimiter` which consistently reproduces the issue.

As a fix I simply check if there are zero parts and add an empty part
before trying to finalize the upload.

This PR also adds the AWS_ENDPOINT env to tests to allow testing locally
against Minio