ACPICA: Fix to allow zero-length ASL field declarations

Allows null field list in Field(), BankField(), and IndexField().

2.6.26-rc1 regression: ACPI fails to load SDT. - Dell M1530
http://bugzilla.kernel.org/show_bug.cgi?id=10606

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>