Geo: PruneEventLogWorker fails when `event_log_min_id` is nil
Seen in staging: https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/638239
NoMethodError: undefined method `-' for nil:NilClass
from geo/prune_event_log_service.rb:44:in `prune!'
from geo/prune_event_log_service.rb:27:in `block (2 levels) in execute'
from geo/prune_event_log_service.rb:23:in `each'
from geo/prune_event_log_service.rb:23:in `reduce'
from geo/prune_event_log_service.rb:23:in `block in execute'
from exclusive_lease_guard.rb:29:in `try_obtain_lease'
from geo/prune_event_log_service.rb:20:in `execute'
from geo/prune_event_log_worker.rb:26:in `perform'
from sidekiq/processor.rb:185:in `execute_job'
from sidekiq/processor.rb:167:in `block (2 levels) in process'
from sidekiq/middleware/chain.rb:128:in `block in invoke'
from gitlab/metrics/sidekiq_middleware.rb:15:in `block in call'
from gitlab/metrics/transaction.rb:55:in `run'
from gitlab/metrics/sidekiq_middleware.rb:15:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from gitlab/sidekiq_status/server_middleware.rb:7:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from gitlab/sidekiq_middleware/correlation_logger.rb:10:in `block in call'
from gitlab/correlation_id.rb:15:in `use_id'
from gitlab/sidekiq_middleware/correlation_logger.rb:9:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from gitlab/sidekiq_middleware/batch_loader.rb:7:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from gitlab/sidekiq_middleware/request_store_middleware.rb:8:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from gitlab/sidekiq_middleware/shutdown.rb:54:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from raven/integrations/sidekiq.rb:9:in `call'
from sidekiq/middleware/chain.rb:130:in `block in invoke'
from sidekiq/middleware/chain.rb:133:in `invoke'
from sidekiq/processor.rb:166:in `block in process'
from sidekiq/processor.rb:137:in `block (6 levels) in dispatch'
from sidekiq/job_retry.rb:108:in `local'
from sidekiq/processor.rb:136:in `block (5 levels) in dispatch'
from sidekiq/rails.rb:43:in `block in call'
from active_support/execution_wrapper.rb:85:in `wrap'
from active_support/reloader.rb:68:in `block in wrap'
from active_support/execution_wrapper.rb:85:in `wrap'
from active_support/reloader.rb:67:in `wrap'
from sidekiq/rails.rb:42:in `call'
from sidekiq/processor.rb:132:in `block (4 levels) in dispatch'
from sidekiq/processor.rb:243:in `stats'
from sidekiq/processor.rb:127:in `block (3 levels) in dispatch'
from gitlab/sidekiq_logging/structured_logger.rb:16:in `call'
from sidekiq/processor.rb:126:in `block (2 levels) in dispatch'
from sidekiq/job_retry.rb:73:in `global'
from sidekiq/processor.rb:125:in `block in dispatch'
from sidekiq/logging.rb:48:in `with_context'
from sidekiq/logging.rb:42:in `with_job_hash_context'
from sidekiq/processor.rb:124:in `dispatch'
from sidekiq/processor.rb:165:in `process'
from sidekiq/processor.rb:83:in `process_one'
from sidekiq/processor.rb:71:in `run'
from sidekiq/util.rb:16:in `watchdog'
from sidekiq/util.rb:25:in `block in safe_thread'