smpp: Only write the systemd-id if it is not empty

system_id is a char array, which makes the NULL check a tautology,
so we should check with strlen if the string is empty or not.

This is fixing the "write" command of VTY that would otherwise
create a config file that can not be parsed.
1 file changed
tree: b04d0e28b3376efdfd4b290d07732270cc63a23f
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README