fixes a problem where an emptied cache from update_images won't be repop...

when calling update_images, the cache is deleted, but not the element uuids.

The store then just checks if the element has a uuid and returns the (now empty) cache value, rather than putting the required data back into the store. If you call a foundation('interchange', 'reflow') after adding images, updating effectively deletes everything that's gone before and none of the interchange images work. 

This version checks to see if the cache value actually exists before returning.