Disable Sidekiq feature flag check if features table does not exist

The GitLab Development Kit initialization failed because the Sidekiq
initializer was attempting to look up a feature flag when the `features`
table hadn't been created yet.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54718

config/initializers/sidekiq.rb

@@ -39,7 +39,7 @@ Sidekiq.configure_server do |config|
     ActiveRecord::Base.clear_all_connections!
   end
 
-  if Feature.enabled?(:gitlab_sidekiq_reliable_fetcher)
+  if Feature::FlipperFeature.table_exists? && Feature.enabled?(:gitlab_sidekiq_reliable_fetcher)
     Sidekiq::ReliableFetcher.setup_reliable_fetch!(config)
   end