For source installs, running `rake gitlab:setup` will fail with this
error:

```

$ bundle exec rake gitlab:setup RAILS_ENV=production
rake aborted!
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: relation
"features" does not exist LINE 8: WHERE a.attrelid =
'"features"'::regclass
```

This happens because the setup Rake task does the following:

1. Checks that Gitaly servers are up
2. The gRPC message attempts to send the activated feature flags
in the metadata.
3. However, since the `features` table doesn't exist, this fails.

To fix this, we just return an empty set if neither the database nor the
`features` table exists.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/212759