Prevent errors on deploy:assets:clean_expired when empty manifests exist on server.

When running `deploy:assets:clean_expired` I got an error ```capistrano-2.15.4/lib/capistrano/recipes/deploy/assets.rb:28:in `parse_manifest': undefined method `to_a' for false:FalseClass (NoMethodError)```

This occurred because we had some empty manifest files on the server.

This adds a checks that the content of the manifests is not empty before it tries to parse the manifests.