static $condition_map not properly initialized, cache performance hit

We couldn't figure out why memcache was being hit 11 times by context_condition_map() when a static variable was being used to store this data per page load. What happened was that no items were set to $condition_map by the foreach loop [in this case array_keys(context_conditions()) was empty], and thus the static variable remained empty.

By initializing the array properly, we avoid multiple cache hits.