FEATURE: blank global settings should not shadow

Due to https://github.com/docker/docker/issues/9298 it is a huge pain
to remove ENV vars when composing images, allow us to simply treat "blank"
as a ENV var that is not being shadowed. In general we always supply a
value to ENV vars we are shadowing.

spec/components/site_setting_extension_spec.rb

@@ -447,6 +447,18 @@ describe SiteSettingExtension do
       end
     end
 
+    context "with blank global setting" do
+      before do
+        GlobalSetting.stubs(:nada).returns('')
+        settings.setting(:nada, 'nothing', shadowed_by_global: true)
+        settings.refresh!
+      end
+
+      it "should return default cause nothing is set" do
+        expect(settings.nada).to eq('nothing')
+      end
+    end
+
     context "with global setting" do
       before do
         GlobalSetting.stubs(:trout_api_key).returns('purringcat')