Remove checking config value with empty() (only supported in PHP 5.5+)

Hi, 

I've noticed this when tried to set up Codex on the VM we use in my office (currently bound to PHP 5.4.x), got this error message: 

```
Arbitrary expressions in empty() are allowed in PHP 5.5 only
```
After changing this line, Codex works fine with PHP 5.4 too.