NoMethodError: undefined method `each' for nil:NilClass

ApplicationRecord.retry_record_not_unique returns nil when it fails with ActiveRecord::RecordNotUnique more than once, which can happen in case of a race condition involving 3+ jobs trying to process entries that maps to the same tenant/integration type. Particularly, this makes https://github.com/3scale/zync/blob/d1d6360de9064adcf5e0e3c7b810b381edc4a87a/app/jobs/process_entry_job.rb#L27 to fail with:

NoMethodError: undefined method `each' for nil:NilClass