smpp_vty: Ensure to not print 'system-id' in openbsc.cfg if its empty

... as an empty system-id would render the openbsc.cfg unparseable on
next openbsc start.
1 file changed
tree: d8d23f3cfd0a152e296816bee59e68e87bf3dba2
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README