xdgmime: Finer handling for cases where mmap() is not available

Allocate an empty cache object, check cache objects for being empty
before using them.
Otherwise the code will re-read cache every 5 seconds, as NULL cache
does not trigger the code that stores mtime, which makes the cache
file appear modified/unloaded permanently.

https://bugzilla.gnome.org/show_bug.cgi?id=735696