Fix rake gitlab:setup failing on new installs

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 the features table doesn't exist. Note that in Rails 6 the table_exists? call is cached: https://github.com/rails/rails/blob/f3ebd0ad2252fec1728ab10275e3f14c79220d25/activerecord/lib/active_record/model_schema.rb#L340

Closes #212759 (closed)

Benchmarks

No signficant performance change:

Calculating -------------------------------------
                orig     2.745k i/100ms
                 new     2.989k i/100ms
-------------------------------------------------
                orig     31.731k (±16.7%) i/s -    153.720k
                 new     31.976k (±15.8%) i/s -    155.428k

Comparison:
                 new:    31975.6 i/s
                orig:    31731.2 i/s - 1.01x slower