edit-util: clean up run_editor() a bit

edit-util: clean up run_editor() a bit

- Add missing assertions
- Close all fds before spawning editor
- Use FOREACH_STRING() + empty_to_null() where appropriate
  Note that this slightly changes the behavior, in that
  empty envvars would be treated as unset and we'd try
  the next candidate. But the new behavior is better IMO.