Fixed error when serving public images from servePublicFile middleware

no issue

- when `servePublicFile` middleware serves an image it resulted in a "Cannot set headers after they are sent to the client" error because `next()` was erroneously called for successful requests which then tripped the `prettyUrls` middleware which tries to perform a redirect
- only calling `next()` when an error is present allows errors to be picked up by later middleware but successful requests end in the `servePublicFile` middleware

core/server/web/shared/middlewares/serve-public-file.js

@@ -30,7 +30,9 @@ function createPublicFileMiddleware(file, type, maxAge) {
                     }));
                 }
 
-                return next(err);
+                if (err) {
+                    return next(err);
+                }
             });
         }