object_store: Fetching empty ranges raises error

**Describe the bug**
Fetching empty ranges raise an error instead of succeeding.

**To Reproduce**
Perform `get_range(path, 0..0)` on the object store.

`error: "object-store error: Generic S3 error: Range started at 0 and ended at 0"`

**Expected behavior**
It should succeed and return an empty `Bytes` object

**Additional context**

Start at the object store's get_range(path, 0..0) entry point and reproduce the reported S3 range error. Trace the empty-range handling and add or update coverage for this case; done means the call succeeds and returns an empty Bytes object.

Written by the indexing model from the issue text.