Unnecessary DB query and cache_set when rules are empty

In function _css_injector_load_rule, empty($cache->data)evaluates to FALSE if cache_get('css_injector:rules') returns an empty array.

That leads to the execution of the second part of the structure, which does an unnecessary DB query and cache_set(), which in turn have a negative impact on performance.

I would suggest to remove that condition, or replace it by is_array($cache->data), though I am not sure why it was added in the first place.