REFACTOR: Clean up parameterized title

Follow up to [FIX: Empty backup names with unicode site titles][1]

- Use .presence - "It's cleaner"
- Update spec to use System.system_user so it is more readable

[1]: c8661674d4

spec/lib/backup_restore/backuper_spec.rb

@@ -5,14 +5,14 @@ require 'rails_helper'
 describe BackupRestore::Backuper do
   it 'returns a non-empty parameterized title when site title contains unicode' do
     SiteSetting.title = 'Ɣ'
-    backuper = BackupRestore::Backuper.new(-1)
+    backuper = BackupRestore::Backuper.new(Discourse.system_user.id)
 
     expect(backuper.send(:get_parameterized_title)).to eq("discourse")
   end
 
   it 'returns a valid parameterized site title' do
     SiteSetting.title = "Coding Horror"
-    backuper = BackupRestore::Backuper.new(-1)
+    backuper = BackupRestore::Backuper.new(Discourse.system_user.id)
 
     expect(backuper.send(:get_parameterized_title)).to eq("coding-horror")
   end