APC: Set ttl to 0 when ttl is not given

Summary:
Passing null as a value for parameter $ttl is deprecated.
Omitting the parameter is considered to be equivalent to 0.

Bug: T15064

Test Plan: * Looked in the deprecation log and did not see a deprecation message from apc.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25833