Rpc: Make GetSerialNumber optional (#24851)

GetSerialNumber previously returned an error if not provided, this
is optional in the spec and should just not return the value if it
is not available.

examples/common/pigweed/rpc_services/Device.h

@@ -372,7 +372,7 @@ class Device : public pw_rpc::nanopb::Device::Service<Device>
         if (DeviceLayer::GetDeviceInstanceInfoProvider()->GetSerialNumber(response.serial_number, sizeof(response.serial_number)) !=
             CHIP_NO_ERROR)
         {
-            return pw::Status::Internal();
+            response.serial_number[0] = '\0'; // optional serial field not set.
         }
 
         // Create buffer for QR code that can fit max size and null terminator.