ACPICA: Add support for zero-length buffer-to-string conversions

Allow zero length strings during interpreter buffer-to-string
conversions. For example, during the ToDecimalString and
ToHexString operaters, as well as implicit conversions. Fiodor
Suietov.  ACPICA BZ 585.

http://www.acpica.org/bugzilla/show_bug.cgi?id=585

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>