platform/x86: lg-laptop: fix %s null argument warning

W=1 warns about null argument to kprintf:
warning: ‘%s’ directive argument is null [-Wformat-overflow=]
pr_info("product: %s  year: %d\n", product, year);

Use "unknown" instead of NULL.
Signed-off-by: Gergo Koteles <soyer@irl.hu>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Link: https://lore.kernel.org/r/33d40e976f08f82b9227d0ecae38c787fcc0c0b2.1712154684.git.soyer@irl.huReviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

drivers/platform/x86/lg-laptop.c

@@ -736,7 +736,7 @@ static int acpi_add(struct acpi_device *device)
 		default:
 			year = 2019;
 		}
-	pr_info("product: %s  year: %d\n", product, year);
+	pr_info("product: %s  year: %d\n", product ?: "unknown", year);
 
 	if (year >= 2019)
 		battery_limit_use_wmbb = 1;