Use guard clause

middleman-core/lib/middleman-core/sitemap/extensions/proxies.rb

@@ -67,11 +67,9 @@ module Middleman
             mime_type = super
             return mime_type if mime_type
 
-            if proxy?
-              proxied_to_resource.content_type
-            else
-              nil
-            end
+            return proxied_to_resource.content_type if proxy?
+
+            nil
           end
         end