[faiproject/fai] 120b39: write $SERVER only if string is non-epmty

[Mrfai]

  Branch: refs/heads/master
  Home:   https://github.com/faiproject/fai
  Commit: 120b39a65f12568344fc43f9f1076222f3624762
      https://github.com/faiproject/fai/commit/120b39a65f12568344fc43f9f1076222f3624762
  Author: Thomas Lange <lange at debian.org>
  Date:   2023-05-23 (Tue, 23 May 2023)

  Changed paths:
    M lib/get-boot-info

  Log Message:
  -----------
  write $SERVER only if string is non-epmty

the problem is that $SERVER is almost always empty, see
https://github.com/faiproject/fai/pull/118
for more info about this bug