Revert changes to `FileStore::S3Store#path_for` in f0620e7118.

There are some places in the code base that assumes the method should
return nil.

app/controllers/uploads_controller.rb

@@ -98,7 +98,7 @@ class UploadsController < ApplicationController
       if Discourse.store.internal?
         send_file_local_upload(upload)
       else
-        redirect_to Discourse.store.path_for(upload)
+        redirect_to upload.url
       end
     else
       render_404

lib/file_store/s3_store.rb

@@ -102,12 +102,7 @@ module FileStore
 
     def path_for(upload)
       url = upload&.url
-
-      if url && url[/^\/[^\/]/]
-        FileStore::LocalStore.new.path_for(upload)
-      else
-        url
-      end
+      FileStore::LocalStore.new.path_for(upload) if url && url[/^\/[^\/]/]
     end
 
     def cdn_url(url)