Fallback to site_hostname when site_title is empty (#4394)

app/helpers/instance_helper.rb

@@ -2,7 +2,7 @@
 
 module InstanceHelper
   def site_title
-    Setting.site_title.to_s
+    Setting.site_title.presence || site_hostname
   end
 
   def site_hostname

spec/helpers/instance_helper_spec.rb

@@ -19,7 +19,7 @@ describe InstanceHelper do
     it 'returns empty string when Setting.site_title is nil' do
       Setting.site_title = nil
 
-      expect(helper.site_title).to eq ''
+      expect(helper.site_title).to eq 'cb6e6126.ngrok.io'
     end
   end