unit-file: avoid (null) in debugging logs
    
    The variable `inst` was set to NULL by TAKE_PTR().
    
    This fixes the following log message:
    ```
    systemd[1]: Unit getty@tty2.service has alias (null).
    ```
    
    (cherry picked from commit 7c35b78a0b96085e3d634542212c5521bc2a2f21)