Return empty hash when a metric returns empty array

[kangkyu]

Sometimes I experience edge cases, once I found we received an empty array for page_fans_gender_age metric when fetch it from a small and new Facebook page. And current code returns nil in that case, that's probably not what we want.

[coveralls]

Coverage decreased (-26.4%) to 71.652% when pulling e6971c2 on kangkyu:empty_array_empty_hash into ef8d34d on Fullscreen:master.

[kangkyu]

https://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings you need set encrypted env variables in travis.yml file to use them in pull requests from fork.

I'll close this and reopen another pull request from origin.