subsys/fs/fatfs: correct misinterpretation of FatFs API

The FatFs f_readdir function returns OK with a zero-length file name
when the last directory entry has been found.  The Zephyr wrapper
unconditionally accessed fields that are left uninitialized in that
situation, propagating garbage values to the caller.

Avoid referencing uninitialized fields of the output structure in this
case.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>