Support for read operations on /dev/null as a Service

This is an enhancement request for DaaS to support read operation.

On many traditional Unix-like systems, /dev/null is not only a trash bin but a handy source of EOFs or an empty file.
Supporting read operation on /dev/null as a Service makes it easy for the users of on-premises /dev/null's to migrate to the DaaS.

With this enhancement you can create an empty file using DaaS:
```sh
wget -O new_empty_file http://.../dev/null
```