ACPICA: AML Debug Object: Don't ignore output of zero-length strings

The implementation previously ignored null strings (""), but
these could be important, especially for debug.

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