Currently, the comparison function only takes into account
access_last_time if it's set for both boxes. If one of them has it
unset, it falls back to using name comparison to sort the boxes.

This means if the user has a box called 'A' and one called 'B', A
having never been used and B being regularly used, A will always
be sorted before B regardless of their access_last_time fields.

This commit ensures that boxes which have their access_last_time field
set are always sorted before the boxes which don't have it set.