fix link_tree argument must be a directory error

[kp-cat]

When I run bundle exec rspec I get this error:

FeatureFlags returns 404 on the market run endpoint
     Failure/Error: //= link_tree ../../../vendor/javascript .js
     
     ActionView::Template::Error:
       link_tree argument must be a directory
     # ./app/assets/config/manifest.js:4
     # ------------------
     # --- Caused by: ---
     # Sprockets::ArgumentError:
     #   link_tree argument must be a directory
     #   ./app/assets/config/manifest.js:4

I created an empty .keep file in that folder to fix this issue.