RepositoryUpdateMirrorWorker attempts to add null import_url

From Sentry: https://sentry.gitlap.com/gitlab/gitlabcom/issues/5787/

Backtrace:

TypeError: wrong argument type nil (expected String)
  from gitlab_git/repository.rb:776:in `create'
  from gitlab_git/repository.rb:776:in `remote_add'
  from lib/gitlab/metrics/instrumentation.rb:153:in `remote_add'
  from app/models/repository.rb:213:in `add_remote'
  from app/models/repository.rb:931:in `fetch_upstream'
  from app/models/project.rb:581:in `fetch_mirror'
  from app/services/projects/update_mirror_service.rb:14:in `block in execute'
  from app/services/projects/update_mirror_service.rb:65:in `call'
  from app/services/projects/update_mirror_service.rb:65:in `update_tags'
  from app/services/projects/update_mirror_service.rb:13:in `execute'
  from lib/gitlab/metrics/instrumentation.rb:153:in `execute'
  from app/workers/repository_update_mirror_worker.rb:13:in `perform'
  from sidekiq/processor.rb:152:in `execute_job'
  from sidekiq/processor.rb:134:in `block (2 levels) in process'
  from sidekiq/middleware/chain.rb:128:in `block in invoke'
  from lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
  from sidekiq/middleware/chain.rb:130:in `block in invoke'
  from lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
  from sidekiq/middleware/chain.rb:130:in `block in invoke'
  from lib/gitlab/metrics/sidekiq_middleware.rb:11:in `block in call'
  from lib/gitlab/metrics/transaction.rb:45:in `run'
  from lib/gitlab/metrics/sidekiq_middleware.rb:11:in `call'
  from sidekiq/middleware/chain.rb:130:in `block in invoke'
  from sidekiq/middleware/server/active_record.rb:6:in `call'
  from sidekiq/middleware/chain.rb:130:in `block in invoke'
  from sidekiq/middleware/server/retry_jobs.rb:74:in `call'
  from sidekiq/middleware/chain.rb:130:in `block in invoke'
  from sidekiq/middleware/server/logging.rb:11:in `block in call'
  from sidekiq/logging.rb:32:in `with_context'
  from sidekiq/middleware/server/logging.rb:7:in `call'
  from sidekiq/middleware/chain.rb:130:in `block in invoke'
  from sidekiq/middleware/chain.rb:133:in `call'
  from sidekiq/middleware/chain.rb:133:in `invoke'
  from sidekiq/processor.rb:129:in `block in process'
  from sidekiq/processor.rb:168:in `stats'
  from sidekiq/processor.rb:128:in `process'
  from sidekiq/processor.rb:80:in `process_one'
  from sidekiq/processor.rb:68:in `run'
  from sidekiq/util.rb:17:in `watchdog'
  from sidekiq/util.rb:25:in `block in safe_thread'

{
args: [
1073469
], 
class: RepositoryUpdateMirrorWorker, 
created_at: 1465935033.846522, 
enqueued_at: 1465938073.5967755, 
error_class: TypeError, 
error_message: wrong argument type nil (expected String), 
failed_at: 1465935035.4411042, 
jid: 333e29fc040a33befcbc2a39, 
queue: gitlab_shell, 
retried_at: 1465938074.1930304, 
retry: True, 
retry_count: 7
}