Project transfer: nil label ID in Lables::FindOrCreateService

https://sentry.gitlab.net/gitlab/gitlabcom/issues/817139/

NoMethodError: undefined method `id' for nil:NilClass
  labels/transfer_service.rb:71:in `find_or_create_label!'
    new_label.id
  gitlab/metrics/instrumentation.rb:161:in `block in find_or_create_label!'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `find_or_create_label!'
    .measure { super }
  labels/transfer_service.rb:20:in `block (2 levels) in execute'
    new_label_id = find_or_create_label!(label)
...
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NoMethodError: undefined method `id' for nil:NilClass