ACPICA: Debug object: Fix output for a NULL object

ACPICA commit e4414f067c726ef746dac990c0bae7f433045843

Was broken by addition of "null string" feature to emit
simple blank lines.

Link: https://github.com/acpica/acpica/commit/e4414f06


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