Provide a void Services instance as fallback

Summary:
The configuration key security.gate.credentials allows to define
a credentials.json file with the list of services credentials.

When the file doesn't exist, instead of deserializing this file,
we provide an empty Services instance.

This fixes a regression introduced at D180.

Test Plan: phpunit

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D196