FIX: Empty backup names with unicode site titles

If a site title contains unicode it may end up with an empty backup
filename because of the rails `parameterize` method we are calling.

This fix ensures that the backup filenames default to "discourse" if the
parameterized site title is empty.

Bug reported [here][1].

[1]: https://meta.discourse.org/t/backup-checksum-and-backup-name-missing-when-unicode-site-name/123192?u=blake

spec/lib/backup_restore/backuper_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+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)
+
+    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)
+
+    expect(backuper.send(:get_parameterized_title)).to eq("coding-horror")
+  end
+end