bootctl: do not show an empty () for ESP partition

bootctl: do not show an empty () for ESP partition

Before:
```
$ run0 systemd-nspawn -xD / --private-network --bind=/sys/firmware/efi/efivars --bind=/boot -- build/bootctl --variables=yes --no-pager
(snip)
Boot Loader Entry Locations:
          ESP: /boot ()
       config: /boot//loader/loader.conf
     XBOOTLDR: /boot ($BOOT)
        token: fedora
(snip)
```

After:
```
run0 systemd-nspawn -xD / --private-network --bind=/sys/firmware/efi/efivars --bind=/boot -- build/bootctl --variables=yes --no-pager
(snip)
Boot Loader Entry Locations:
          ESP: /boot
       config: /boot//loader/loader.conf
     XBOOTLDR: /boot ($BOOT)
        token: fedora
(snip)
```

This also moves spurious position of new line in each output.