acpi: property: Let args be NULL in __acpi_node_get_property_reference

acpi: property: Let args be NULL in __acpi_node_get_property_reference

fwnode_get_property_reference_args() may not be called with args argument
NULL on ACPI, OF already supports this. Add the missing NULL checks and
document this.

The purpose is to be able to count the references.

Fixes: 977d5ad39f3e ("ACPI: Convert ACPI reference args to generic fwnode reference args")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20231109101010.1329587-2-sakari.ailus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>