fix: update custom_config method to return an empty hash by default

[rwc9u]

Datadog tracing integration with ci_queue and datadog-ci-rb assumes that redis custom config is a hash. This PR defaults ci_queue's custom_config method to return an empty hash if debug logging is not set.

/Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/datadog-2.22.0/lib/datadog/tracing/contrib/redis/trace_middleware.rb:77:in `resolve': undefined method `[]' for nil (NoMethodError)

            custom = redis_config.custom[:datadog] || {}
                                        ^^^^^^^^^^
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/datadog-2.22.0/lib/datadog/tracing/contrib/redis/trace_middleware.rb:16:in `call'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-client-0.25.3/lib/redis_client.rb:315:in `block in call_v'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-client-0.25.3/lib/redis_client.rb:721:in `ensure_connected'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-5.4.1/lib/redis/client.rb:91:in `ensure_connected'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-client-0.25.3/lib/redis_client.rb:314:in `call_v'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-5.4.1/lib/redis/client.rb:97:in `call_v'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-5.4.1/lib/redis.rb:154:in `block in send_command'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-5.4.1/lib/redis.rb:153:in `synchronize'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-5.4.1/lib/redis.rb:153:in `send_command'
	from /Users/rob.christie/.asdf/installs/ruby/3.3.8/lib/ruby/gems/3.3.0/gems/redis-5.4.1/lib/redis/commands/keys.rb:280:in `exists?'
	from /Users/rob.christie/dev/ci-queue/ruby/lib/ci/queue/redis/worker.rb:77:in `retrying?'
	from /Users/rob.christie/dev/ci-queue/ruby/lib/rspec/queue.rb:461:in `run_specs'