parse_subscription: Handle case where next_payment_date is the empty

string. This happens when a subscription is cancelled.

Signed-off-by: Birin Sanchez <birin.sanchez@puri.sm>
1 job for 407_canceled_subs_crash in 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Deb
passed #48297
liberty
deb

00:00:42