qemu-img backed over https fails on zero-length file

When creating new disk with qemu-img backed by file accessed over HTTPS and the backing file has zero length, qemu-img fails with uninformative error:

        qemu-img: disk.qcow2: CURL: Error opening file:

The error message should either be improved, or the operation on zero-length file should be allowed. Some other backends, as e.g. raw backend for regular files, seem to allow empty files.