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: 977d5ad ("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>

Author: Sakari Ailus

drivers/acpi/property.c

@@ -852,6 +852,7 @@ static int acpi_get_ref_args(struct fwnode_reference_args *args,
  * @index: Index of the reference to return
  * @num_args: Maximum number of arguments after each reference
  * @args: Location to store the returned reference with optional arguments
+ *	  (may be NULL)
  *
  * Find property with @name, verifify that it is a package containing at least
  * one object reference and if so, store the ACPI device object pointer to the
@@ -908,6 +909,9 @@ int __acpi_node_get_property_reference(const struct fwnode_handle *fwnode,
 		if (!device)
 			return -EINVAL;
 
+		if (!args)
+			return 0;
+
 		args->fwnode = acpi_fwnode_handle(device);
 		args->nargs = 0;
 		return 0;