(s)fdisk: avoid unneeded empty lines with '--list'

(s)fdisk: avoid unneeded empty lines with '--list'

On my system, I got two superfluous empty lines because /dev/sr0 didn't
contain a medium. Refactor the code to handle the seperator within
print_device_pt() and print it only when assigning the device worked.
This unifies handling between print_all_devices_pt and (s)fdisk because
the latter did not consider the return code for the seperator while the
former did. Also, it saves some lines of code.

Addresses: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814184 (first part)
Signed-off-by: Wolfram Sang <wsa@kernel.org>