cachedir is used to determine where cache files are stored.
the empty directory will ends up to put them under the top of
directory of XDG_CACHE_HOME. that messes it up and must be avoided.
As a note, if you want to read/write something from the top of
XDG_DATA_HOME (dir) and/or XDG_CACHE_HOME (cachedir),
put "." instead of the empty.
https://gitlab.freedesktop.org/fontconfig/fontconfig/issues/180
Akira TAGOH