FIX: nil and blank string should not override a site setting

lib/site_setting_extension.rb

@@ -121,7 +121,8 @@ module SiteSettingExtension
       # exists it will be used instead of the setting and the setting will be hidden.
       # Useful for things like API keys on multisite.
       if opts[:shadowed_by_global] && GlobalSetting.respond_to?(name)
-        unless (val = GlobalSetting.send(name)) == ''.freeze
+        val = GlobalSetting.send(name)
+        unless val.nil? || (val == ''.freeze)
           hidden_settings << name
           shadowed_settings << name
           current_value = val