CopyNandFile must not create empty files on the destination filesystem
if the source file doesn't exist.
Otherwise, this can lead to an empty Mii database being created in the
session Wii root if there's no database in the configured Wii root and
netplay or Movie is used -- that database would then be copied back to
the configured root, which causes games like MKW to complain about
corrupted Mii data even when the player has stopped using netplay.
This commit also simplifies CreateFullPath usage.
There's no need to manually extract the directory from the path,
FS::CreateFullPath does it automatically just like File::CreateFullPath
47ee5ac284