ACPI: avoid empty file name in sysfs

Since commit bc45b1d3

 acpi tables are
allowed to have an empty signature and /sys/firmware/acpi/tables uses the
signature as filename.  Applications using naive recursion through /sys
loop forever.  A possible solution would be: (replacing the zero length
filename with the string "NULL")

http://bugzilla.kernel.org/show_bug.cgi?id=11539

Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>