ServiceConnection: Make vpd serial_number optional

Making this field required led to issues with test devices. This changes
it to optional like many other fields.

Bug: chromium:1146143
Change-Id: Ie0bdfe38462aef987021c42eec20749255d5ddf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536150Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Mason Wilde <masonwilde@google.com>
Cr-Commit-Position: refs/heads/master@{#827343}

chrome/browser/chromeos/policy/status_collector/device_status_collector.cc

@@ -1240,8 +1240,10 @@ class DeviceStatusCollectorState : public StatusCollectorState {
               system_status_out->set_vpd_sku_number(
                   system_info->product_sku_number.value());
             }
-            system_status_out->set_vpd_serial_number(
-                system_info->product_serial_number);
+            if (system_info->product_serial_number.has_value()) {
+              system_status_out->set_vpd_serial_number(
+                  system_info->product_serial_number.value());
+            }
           }
           if (report_system_info) {
             system_status_out->set_marketing_name(system_info->marketing_name);

chromeos/services/cros_healthd/public/mojom/cros_healthd_probe.mojom

@@ -474,7 +474,7 @@ struct SystemInfo {
   // The product SKU number.
   string? product_sku_number;
   // The product serial number.
-  string product_serial_number;
+  string? product_serial_number;
   // Contents of CrosConfig in /arc/build-properties/marketing-name.
   string marketing_name;
   // The BIOS version.