Fix type error when data isn't cached in ProjectsMap

Summary:
When the cache doesn't contain a map, and loadFromCache method were
called, the map private property (with [] as default value) received
null value instead.

This fixes allow the offsetExists method to work in this scenario.

Test Plan: Run cache tests from D324

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D325