Fixing Memcache::get() returns NULL for an empty string problem

Summary: Fixes #4175
Closes https://github.com/facebook/hhvm/pull/4232

Reviewed By: @JoelMarcey

Differential Revision: D1676834

Signature: t1:1676834:1415918294:5a1c59a5a478e9085cc4687475ae2c58944bbb14