Fix adding the product_id to account based charges (when services are not active)

app/Console/Commands/Intuit/InvoiceAdd.php

@@ -70,13 +70,13 @@ class InvoiceAdd extends Command
 				sprintf('%s.%s.%s.%s',
 					$item->item_type_name,
 					$item->price_base,
-					$item->product->provider_ref($to->provider),
+					$item->product?->provider_ref($to->provider),
 					$item->taxes->pluck('description')->join('|'))) as $os)
 		{
 			$key = $os->first();
 
 			// Some validation
-			if (! ($ref=$key->product->provider_ref($to->provider))) {
+			if (! ($ref=$key->product?->provider_ref($to->provider))) {
 				$this->error(sprintf('Accounting not defined in product [%d]',$key->product_id));
 
 				return self::FAILURE;