pubsub: accept nil duration from server
pubsub: accept nil duration from server

Although the service should never give us a nil Subscription.MessageRetentionDuration,
the emulator still does.

Fixes #847.

Change-Id: Ic68e974614be4f2f4d4c757c0d07b63650d72947
Reviewed-on: https://code-review.googlesource.com/22570
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go