ACPICA: iASL: Fix for WPBT table with no command-line arguments

Handle the case where the Command-line Arguments table field
does not exist.

ACPICA commit d6487164497fda170a1b1453c5d58f2be7c873d6

Link: https://github.com/acpica/acpica/commit/d6487164


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>