NoMethodError when DPLA job runs in a month with no DPLA load

Sentry Issue: DLG-ADMIN-38

NoMethodError: undefined method `-' for nil:NilClass (NoMethodError)

  if today == target_date - 1 && !force_run
                          ^
  lib/tasks/feed_the_dpla.rake:147:in `block in <main>'
    if today == target_date - 1 && !force_run
...
(16 additional frame(s) were not displayed)