Fix Cache::get()/Cache::section()->get() for empty-like values that are not null

When non-null but empty-like* values are placed in the cache, ````Cache::get()```` or ````Cache::section()->get```` won't return them since the ternary operator used will evaluate them as empty and therefore return null.


* **Empty-like:** http://www.php.net/manual/en/function.empty.php